This is a good post, and I recommend reading the whole thing, but the most important point to me is what do you spend your time doing in-game? And is it fun?
If you expect your players to spend most of their time at the level cap, the post-cap game is rather important, ya know? If most WoW players are at the cap, those raids and/or the PvP better be lots of fun, or else hitting the cap is a suggestion that people quit playing your game. Maybe alts are lots of fun; that’s how we roll in City of Heroes.
That is not a suggestion that you make time-to-cap take longer. If the leveling game is fun, people will make alts to keep playing it. They will keep running your quests for the fun, even without experience points. If not, a longer time-to-cap is bad because it means less fun in your game, although really it means throw away your entire game, because we already have dozens of level-based MMOs, some of which are pretty good.
I’ll rephrase that last point: if your game is 250 hours to the level cap, make sure your game has at least 250 hours of leveling fun, and 1000 would be better so that people can play some alts. If your game is 2000 hours to the level cap (hello classic EQ) … I’m betting there was not 2000 hours of fun, so let’s move on. Once you have players at the level cap, you need enough hours of fun there to keep them playing. And no, taking the same amount of fun and trying to stretch it out by lowering xp/encounter or making you re-run each part until it is not fun does not work: that just increases global suck.