I have been spending my fair share of time lately leveling various alts towards level 30 so they can get their standard land mounts and in all this the question of whether or not boosting would be a faster way to level came up repeatedly.
So I decided to make a quick post on the topic, probably forgetting 50% relevant information and 100% post structure while I write ;)
First of all lets start out with a quick description of what boosting actually is and what forms of boosting there are. Essentially boosting is nothing more than using one of your (usually higher level) friends / guildies or a complete stranger to help you through various passages in the game in order to get the related xp.
That way you can realistically do instances on your level without the risk of dying (your friendly neighbourhood lvl 70 will keep you save) or finish up group quests that you just can't seem to tackle alone.
Boosting comes in multiple flavors.
1. An instance boost where a higher level player walks your lower level character through the instances pretty much doing all the killing and reducing you to a loot-bot.
2. Quest boosting. Quite common in guilds, it involves a higher level player rapidly killing quest mobs for you so you can turn in quests faster.
3. Tag-boosting. Something I don't see very often but it involves you tagging the mob so that you get the experience and then leaving it to the higher level character to finish it of.
We'll leave the rag tag world of pvp boosting / point selling, team trading, win-trading for another post since it's really not my expertise and just focus on how to get that little character on a fast-track to 70.
In order to compare we also have to take a look at regular questing / grinding as well as questing / grinding with a friend:
Quest XP rewards have been on a steady incline. Blizzard is fairly clear in that it prefers us to quest to level rather than to straight out grind and the quest rewards reflect this quite well. You cannot make more xp grinding than you can make questing if you take on quests that are fairly straight forward (the kill x kind of quests).
Due to the increased quest rewards these days the situation is very much so that you will get the most xp by questing / grinding solo. Ideally you would strive for a situation where you do only 'kill x' quests so that you are maximizing your time spent gaining xp. Escort quests and long quest chains generally involve a lot of travel time and tend to slow things down a lot (not to mention failing an escort quest is horribly bad for your xp per hour).
Straight up grinding will never be quite as fast as a combination of questing because you don't get to turn in quests.
Ironically enough buddying up with someone actually slows down your experience gain. The more people in your party, the more the XP will be divided and the more you will have to share quest mobs. Even if you are moving twice as fast you will still have to split the experience and kill twice as many mobs as you had to before just to get the necessary quest item drops.
The more people in your party, the slower things will go.
Taking a loot at boosting, instance boosting sounds like the most interesting boosting options. However due to the mechanic of XP division between a higher level and a low level character you really only get a fraction of the XP from each kill.
This means that while instance boosting is a good way to accumulate gear and random crapola for crafting and vendor selling it really turns out to be more of profit run and really doesn't compare to straight up questing in terms of XP per hour.
Quest boosting on the other hand is a very good way to speed up your experience gain. Basically it involves grouping up with someone who can kill your level mobs in 1 or two hits, setting your loot options to free-for-all and letting the high level buddy go to town killing large amount of quest mobs.
This lowers your experience gain per kill but drastically decreases the time it takes for you to gather up the necessary quest items.
This means you get to hand quests in faster and suffer from fewer 'accidents' to boot (i.e. the occasional death and corpse run, repairs, ammo runs etc.)
Last but not least there's tag boosting. Tag boosting is interesting in that you don't actually group up with your higher level friend. You hit the target mob once or twice and then let the higher level finish it off. That way you get experience credit for the kill (due to tagging) and full looting rights.
The disadvantage is that it requires a lot of coordination. You can't have your high level go to town and mow everything down because you wouldn't get loot or xp.
Tag boosting works quite well in situations where there are few enemy mobs to be tackled (i.e. group quests) but is generally inferior to straight up questing/grinding due to the loss of speed.
So to summarize:
1. More people in your group means lower xp / loot gains
2. Boosting instances is good for gold and items and not so much experience
3. Quest boosting is a very efficient way to get xp but you will go through quests in a zone like a warm knife through butter.
4. Tag boosting can be useful in group quest situations but generally tends to require too much coordination to be efficient.