Keen is pondering people reaching the Guild Wars 2 level cap before the game is even officially released. I can easily imagine the train of thought going on in some players:

1. "I must rush to the level cap, because that is where the real game is."
2. "Yay, I reached the level cap, now let's look at the endgame."
3. "Hmmm, the endgame doesn't offer anything new."
4. "Unsubscribe"

Much has been written about how MMORPGs before, up to and including World of Warcraft, tended to grow over time and peak many months or even years after release, while post-WoW games tend to peak at the end of the first month and are in serious decline 3 months later. I would bet that this trend correlates with the leveling speed of modern games.

Sure, you would get some people bitching and moaning if you released a MMORPG today which took 3 to 6 months to reach the level cap. But as usual what people complain about and what they act upon are two very different things: In a game with slow leveling they would complain and keep playing; in a game with fast leveling they would say, if asked, that this is what they wanted, but they'll unsubscribe after a month or three.

In short: I believe that making MMORPGs with fast leveling is a serious business error.