I was digging through the older posts from wowinsider the other day and ran across this little gem here.
Basically it is a short overview of different stats and who can / can't use them. The list isn't perfect but I certainly think it's worth a glance over whenever you roll a new alt or are just starting off your career in wow.
I wish I had found this list when I started...
All credit goes to Elizabeth Harper over at wowInsider.com, the poor formatting is mine however.
Agility (AGI): Increases ranged and melee attack power (for some classes), increases armor rating, increases dodge chance, increases your chance to score a critical strike with a weapon.
Warriors, Hunters, and Rogues gain 1 ranged attack power per agility.
Hunters, Rogues, and Druids in cat form (yes, only cat form) gain 1 melee attack power per agility.
Attack power increases your damage per second (DPS) by 1 for every 14 attack power.
So who's agility important for?
Hunters: Agility gives you more ranged attack power and critical rating which means more damage. At later levels, you may find gear with straight out attack power and critical rating to be more attractive.
Rogues: Agility is one (of two -- the other being strength) sources of melee attack power for rogues. However, agility also increases critical rating and dodge chance -- the former important for damage and the second for defense.
Druids (cat): Because agility only benefits attack power for cat druids, you might consider strength as an alternative, since it provides attack power in both cat and bear form. However, it doesn't provide the critical rating, armor, or dodge. Cat-form Druids with the Improved Leader of the Pack talent (which heals the Druid whenever the Druid scores a critical strike) will likely be more interested in the boost to crit agility provides.
Warriors: Agility doesn't improve their melee attack power at all, though it will improve their critical rating. However, its defensive aspects are worthwhile -- increased armor and dodge are both good choices for tanking Warriors... but agility gives so little of these things per point that low-level tanks are likely to find more value in straight out stamina while higher level tanks are likely to be more interested in specialized stats like defense, dodge, resilience, etc. (More on those later!)
Paladins: Agility has the same defensive improvements for Paladins as it does for Warriors.
Shamans (enhancement): This may not improve their attack power, but it still improves their chance to score a critical strike. With Flurry (increasing attack speed after a crit) and Unleashed Rage (increasing your party's attack power after a crit) strike, so agility can be key to certain build types -- though at higher levels, you're likely to find gear with straight-out critical rating to be more useful.
Intellect (INT): Increases your mana, increases your chance to score a critical strike with spells, and improves the rate at which you learn weapon skills. For Warlocks it will also increase their pets' intellect (and therefore their pets' mana).
For each point of intellect you have, you gain 15 mana.
The amount of spell critical strike received per point of intellect varies per class and per level. (Though we don't have exact numbers for other levels, expect less intellect to give you more spell crit in the lower levels.)
At level 60...
Warlocks receive 1% spell crit for each 60.6 points of intellect.
Druids receive 1% spell crit for each 60 points of intellect.
Shamans receive 1% spell crit for each 59.5 points of intellect.
Mages receive 1% spell crit for each 59.5 points of intellect.
Priests receive 1% spell crit for each 59.2 points of intellect.
Paladins receive 1% spell crit for each 54 points of intellect.
At level 70...
Warlocks receive 1% spell crit for each 81.92 points of intellect.
Druids, Shamans, Mages, Priests, and Paladins receive 1% spell crit for each 80 points of intellect.
As you might guess from these figures, with the amount of intellect needed to get a single point of spell crit, you'll often find it easier to improve your critical percentage with crit rating gear rather than massive amounts of intellect, especially at level 70.
Several classes can increase their spell damage by a percentage of their intellect, with the appropriate talent selections.
At tier 8 in the Arcane tree, Mages can acquire Mind Mastery, which (for five talent points) increases their spell damage and healing by up to 25% of their intellect.
At tier 8 in the Holy tree, Paladins can acquire Holy Guidance, which (for five talent points) increases their spell damage and healing by up to 35% of their intellect.
At tier 8 in the Restoration tree, Shamans can acquire Nature's Blessing, which (for three talent points) increases their spell damage and healing by up to 30% of their intellect.
At tier 5 in the Balance tree, Druids can acquire Lunar Guidance, which (for three talent points) increases their spell damage and healing by up to 25% of their intellect.
At tier 3 in the Arcane tree, Mages can acquire Arcane Fortitude, which (for one talent point) increases their armor by 50% of their intellect.
At tier 5 in the Elemental tree, Shamans can acquire Unrelenting Storm, which (for five talent points) allows them to regenerate mana equal to 10% of their total intellect every 5 seconds, even while casting. (Read: under the five second rule. See the spirit section for more information on mana regeneration and the five second rule!)
At tier 7 in the Balance tree, Druids can acquire Dreamstate, which (for three talent points) allows them to regenerate mana equal to 10% of their total intellect every 5 seconds, even while casting. (Read: under the five second rule.)
At tier 7 in the Marksmanship tree, Hunters can acquire Careful Aim, which (for three talent points) increases their attack power by 45% of their intellect. (Meaning that each point of intellect also gives the Hunter about half a point of attack power.)
So who's intellect important for?
All casters: Since intellect controls the amount of mana you have, it's important for anyone with a mana bar. For soloing and grouping, you need a big enough mana pool to allow you to continue doing damage (or healing) through an average fight. (After all, mana potions are expensive.) If you find yourself running out of mana mid-fight or stopping to drink between every fight, you may want to consider looking into more intellect gear.
Healers: If you're interested in healing groups, I personally advise weighing intellect fairly heavily up to level 60 or so (when you'll start having easy access to +healing, MP5, and other handy stats). Having a large mana pool will allow you to keep healing through bad pulls or numerous adds, which can occasionally mean the difference between life and death for the party.
Stamina (STA): This one's easy -- your health! For Hunters and Warlocks it will also increase their pets' stamina (and therefore, their pets' health).
For each point of stamina you have, you gain ten hit points.
If you're a tauren, you get 10.5 hit points for each point of stamina.
So who's stamina important for?
Everyone: If you ever get hit by monsters in your adventures throughout Azeroth, stamina will help you out by delaying your inevitable demise. (However, certain classes may favor the "glass cannon" approach to adventuring, in which case health and survivability is less important than dishing out damage and killing the target before it can kill them.)
Hunters & Warlocks: May find stamina more useful than other classes do for the improvement it gives their pets.
Warlocks: Their Life Tap ability turns health into mana (the base spell gives an even return of mana for health, but with two talent points in Improved Life Tap, they'll get more mana for less health). Because they can make this exchange so easily and regenerate their own health with skills like Drain Life and Death Coil, Warlocks will often favor stamina over intellect.
Tanks: Tanking Druids, Warriors, and Paladins will all find that a certain level of stamina is absolutely essential to tanking instances. Healers can only cast heals so quickly, and you're going to need to survive taking hits between those heals. (The exact amount of health needed varies depending on instance -- you'll get a feel for the more you play.)
Spirit (SPI): Increases health regeneration while out of combat and increases mana regeneration while not casting spells. We're primarily going to discuss it for mana regeneration, since there are easy ways to regenerate health out of combat (food, bandages), but few ways to regenerate mana in combat.
Mana begins regenerating via spirit only when you have not cast any spell using mana during the previous 5 seconds. (This is generally referred to as the "five second rule.")
If you haven't cast a spell in the past five seconds, your mana will begin regenerating every two seconds (or every "tick"). The amount regenerated varies based on your class:
Druid (caster): Spirit/4.5 + 15
Druid (feral), Hunter, Paladin, Warlock: Spirit/5 + 15
Mage, Priest: Spirit/4 + 12.5
Shaman: Spirit/5 + 17
Several classes have talents or abilities that allow spirit to improve their mana regeneration while in the five second rule.
At tier 3 in the Restoration tree, Druids can acquire Intensity, which (for three talent points) allows 15% of their mana regeneration to continue while under the five second rule.
At tier 4 in the Arcane tree, Mages can acquire Arcane Meditation, which (for three talent points) allows 15% of their mana regeneration to continue while under the five second rule.
At tier 3 in the Discipline tree, Priests can acquire Meditation, which (for three talent points), allows 15% of their mana regeneration to continue while under the five second rule.
Mages' Mage Armor buff (first available at level 34) allows 30% of their mana regeneration to continue while under the five second rule.
Several classes have talents that allow spirit to improve their spell damage or healing:
At tier 5 in the Holy tree, Priests can acquire Spiritual Guidance, which (for five talent points) increases their spell damage and healing by up to 25% of their total spirit.
At tier 5 in the Discipline tree, Priests can acquire Improved Divine Spirit, which (for two talent points) increases the spell damage and healing of players with the Divine Spirit buff by 10% of their total spirit.
At tier 9 in the Restoration tree, Druids can acquire Tree of Life Form, which (for one talent point) increases healing received by party members within range of the Druid by 25% of the Druid's total spirit.
A couple of spells will vastly increase your normal mana regeneration (read: mana regeneration from spirit) for a brief period of time:
Starting at level 40, Druids have access to Innervate, which will increase the target's mana regeneration by 400% (casting or not casting -- Innervate causes your mana regeneration to disregard the five second rule) for 20 seconds (on a 6 minute cooldown).
Starting at level 20, Mages have access to Evocation, which will increase the Mage's mana regeneration by 1500% for 8 seconds (on an 8 minute cooldown).
So who's spirit important for?
Everyone: In general, spirit's impact on both health and mana regeneration or so minor, that it's not a terribly important stat for anyone. In fact, even Blizzard seems to have caught on to this idea, so you'll usually find poor spirit itemization on better (blue and up) gear. For classes relying on mana, you'll often find MP5 gear to be a better way to regenerate mana over the course of a fight. Depending on how you cast -- for DPS casters who chain-cast spells, MP5 will always provide better mana regeneration than spirit. But for healers who may have breaks between casting, spirit can be a good option. Obviously, if you're a class with a talent that increases your mana regeneration during the five second rule, spirit is of more use, but how useful it is still depends on how much time you spend in and out of the five second rule.
Healers: The usefulness of spirit to a healer depends heavily on casting style. If, while healing your group, you cast spells in spurts and then have several seconds of waiting before you act again, spirit can be a useful way to regenerate mana. (If you take this tactic, I advise finding yourself a high-spirit staff enchanted with +20 spirit to equip during your downtime -- since you can swap weapons during combat. Trust me, it makes a difference!)
Anyone who might be Innervavted: In raids and groups, Innervates are often saved for the healers. In this situation, spirit will play a large role in how much mana you get back. If you see that you've just been Innervated, equip your spirit staff (you have one, right? With a +20 spirit enchant?) and keep casting -- Innervate causes your mana regeneration to ignore the five second rule.
Druids (caster): For a druid speccing in Restoration down to Tree of Life Form, spirit could be a reasonable investment to improve their healing, though at higher levels, +healing gear will likely be easier to obtain
Priests: With Spiritual Guidance and Improved Divine Spirit, you'll find spirit a decent addition to your spell damage and healing. The only problem is finding enough high-spirit items to work these talents to their max.
Mages: Spirit is what makes Evocation and Mage Armor do their thing for your mana regeneration -- so you will find some investment in spirit to be worthwhile. At the very least, look for a high spirit staff to put a +20 spirit enchant on to swap in when you use Evocation!
Strength (STR): Increases your melee attack power and the amount of damage you can block with a shield.
Warriors, Shamans, Druids, and Paladins gain 2 melee attack power per strength.
Rogues, Hunters, Mages, Priests, and Warlocks gain 1 melee attack power per strength.
Remember, attack power increases your damage per second (DPS) by 1 point for every 14 attack power.
At tier 6 in the Feral tree, Druids can acquire Nurturing Instinct, which (for two talent points) improves their +healing by 50% of their strength.
So who's strength important for?
Warriors: This is a warriors' source of attack power -- and, thusly, damage. Arguably the most important stat for a DPS warrior, but also a reasonable stat for Protection spec warriors, as more damage will give them more threat and the additional damage mitigation with your shield doesn't hurt, either.
Shamans: This is where the melee-powerhouse Enhancement Shaman gets their attack power and a good portion of their damage.
Druids (feral): Strength is my stat of choice for feral druids, simply because it works for druids in both bear and cat form which means one less set of gear to carry around. You lose out on extra dodge and critical strike chance by choosing Strength over Agility, but you gain more attack power overall -- and for both forms! Also, for feral Druids playing a hybrid role, with Nurturing Instinct, strength also improves your healing abilities -- meaning that when you shift out of your feral form to cast an emergency heal, your feral strength gear still helps you out.
Rogues: Rogues get just as much attack power from strength as they do from agility, so I'd advise favoring agility for the dodge and critical strike chance.
Paladins: This is the Paladin's only source of attack power, so it's important for Retribution Paladins (to improve their DPS) and Protection Paladins (to improve their DPS and thus their threat as well as increasing their shield's damage mitigation potential). However, at higher levels, Paladins (of all types) are likely to find spell damage gear to be a more effective way to improve their damage output, as so much of their overall damage output is done with Seals, Judgements, and other spells doing holy damage.