From my WoW experience, I've had fun doing both. Leveling my first character was a blast. Seeing new sights, following along interesting questlines, picking up not just new gear, but new abilities and talents.

But things were still pretty good after I hit the level cap. Instead of seeing new zones I was seeing new instances, and instead of new moves, my existing ones were propelled to superhuman power by the fat epics of Karazhan, or at least that's how it made me feel.

Regardless of level, I really do think it's the eventual repetition that gets to you (and me). The raid instance is less fun after you've been through it for the same reasons the starting areas are. I think a lot of the people "rush" to the endgame content in an MMO do so mainly because they've been through all of the startgame and midgame content before. And endgame seems to be the only thing, at least in WoW, to get content patches anymore.