Everything gets bigger.
This is an axiom that has held true for most games in the game industry but especially MMORPGs. Patches and expansions are rolled out in order to give the player more to do and to keep the money rolling in for those that design/create the game.
Everything getting bigger has held true even in the days of the old dungeon and dragons. I remember the days where we would flail around in small groups, slaying the occasional orc or desperately running away from the dragon's cave. I remember those days where level 15 was the ultimate goal and represented a significant portion of power for anyone involved in AD&D.
But it got bigger. In the form of extra rulebooks and other expansions and in the end the old power cap of level 15 was extended for everyone.
And with this growth of the power cap things became skewed. Combat tables and rulesets that were initially designed for level 15 and under simply didn't work anymore for players that literally outgrew the content and were well on their way to level 30.
The world initially designed for a modest power cap of level 15 was only able to cope with these new godlike players by throwing new godlike challenges in their face via supplemental rulebooks and yet more expansions.
How do you provide content for godlike players?
How do you provide content for those that have accumulated wealth far beyond what the game was initially designed for?
This is a conundrum that MMORPG designers have been fighting with for ages and thusfar the only answer they could come up with are simple reactive measures to structural problems of the game design.
If people are super rich... then we make stuff super expensive (remember the cost of black dye tubs in ultima online? or say... a wow mammoth mount)
If people are super proficient... then we raise the difficulty of their proficiencies (say hello to another tier of crafting, fishing, cooking etc.)
If people are super powerful... then we simply add creatures that are more powerful than them (I heard the lich king has between 2 and 3 million hit points, assuming we get to fight him)
If people have super gear... then we add gear that is more super (ok fine... better... ).
And once again things get bigger.
Both the designer and the player find themselves stuck in a cycle. The designer raises the power cap and this immediately raises the bar for the player who will always strive to get more wealth, more power and as a result will go out and slay the more powerful creatures that have just been added to the game via the patch.
That illidan chap? He's a pushover now... curator, prince, magtherion you name them... mere jokes that have been relegated to the realm of those that like to solo dungeons with the new gear that has been added in the latest patch.
And as things get bigger, bigger numbers start to cause significant problems. A game that was initially balanced for 60 levels will find it more and more difficult to cope with 10,20,30,40 additional levels and more stats to go along with it. A 10% bonus to stamina may have been insignificant at level 30 but is a huge boost at level 100 where the stats are much greater and percentile scaling goes through the roof.
The problem becomes exponential even with a linear increase in stats across the board and designers/developers will have to jump through flaming hoops to retain even the smallest sliver of balance.
What it comes down to is this: No game system can adequately handle players that are too far outside of it's initial design.
And that's not so far fetched... look at real life. If you apply yourself, work hard or just have natural talent that you can cultivate you may become an Einstein, a Sun tzu, a vincent van gogh but your potential is capped. You will never be a god (i.e. all-powerful) whereas most game systems will lead you from the ordinary to the heroic to true godhood in the end.
If you allow people to grow beyond the hard cap of the initial design you effectively turn a player into an unknown variable. You have no control over this player other than to satisfy his/her need for 'more' and the only way to keep the player happy at that point is to add new, more powerful, bosses and give the player the equivalent of a nuclear bomb to fight them with.
Without a hard cap on player potential you invalidate your carefully crafted gaming environment. People will stumble over themselves trying to get to the end-game ignoring whatever was between just to get to those new, hastily thrown together, bosses and instances that carry the most powerful equipment and yield the highest level of rewards.
As things get bigger, your game system starts to show cracks, glitches and weird effects like sudden problems with burst damage, 100% critical hit chances, massive resistance issues and problems that previously never seemed to be an issue.
Your players are leaving the ruleset, the boundaries of initial design... they have become too powerful, the numbers too large to control. I've seen it happen a dozen times ... games that ended up so skewed that players could do so much damage that an attack was either fully resisted and no damage taken whatsoever or the player was instantly pulverized when hit.
Now that's Epic...
I am watching the numbers increase and I can see the potential problems.
I'll give you one of many examples. Try not to nitpick it too much, it's an example after all: Resistances. With the new wotlk gear its easy for a paladin with a frost resistances aura and about 3-4 pieces of gear to become fully capped in frost resistance. With a little bit of creative gearing and some aura switching the same paladin can probably achieve full frost and full fire resistance at the same time.
Once you reach this point you'll probably have a lot of unhappy mages walking the world (I for one never heard a mage curse more than when most of their spells get resisted).
Certainly this comes at the cost of other stats, but it illustrates that bigger numbers will invariably lead to bigger problems.