Well its a hard line to follow, either you make the game simple to level up and people race through content like WoW and they quit cuase they finish to fast or you make it very hard to level up like EQ1 and people quit because it takes to much of a time commitment. Who can put in 40 hours a week playing a game now days?

I would think the best is to make it simple to level but have some very very hard raids (or in world mobs) that can only be beaten by the top 1 percent of the player base.

Or like suggested make it easy to level but hard and long to add in special AA's or whatever.