There are, as he says, tons of things to do for attunement to most of the endgame dungeons-- including attunements just to get attuned for the attunements. And that's just for mains-- your alts will also have to get attuned if you want to take them to the endgame raids. Considering raid group numbers are lowering (from 40 to 25), that means guilds are going to be slimming down by necessity-- not only will fewer people be able to go, but fewer people will get to go because the attunements will rule a lot of people out.
And this, says Kamikazeelf, will lead to something called "Noob Extinction" (I happen to enjoy the phrase). Basically, if it's so hard to get into these dungeons, and so few people can go, players who don't know what they're doing will quickly get outed. On a 40-man raid, you could afford 10 people standing there and doing half a job. But in a 25 man raid, where a whole guild has to run instances over and over just to get people attuned, the proverbial "fools" just can't be suffered. At the highest instances (and there are a few of them), the rule will be that you're good, or you're gone.
Now, there are, of course, exceptions. Some guilds are friends, and everyone will help everyone else out so everyone has a good time. Some players won't bother with PvE at all, and instead focus characters only on PvP (for my alts, this is probably what I'll do). But the truth is that this will leave some players out in the cold-- both kids who don't know what they're doing, and casual players who can't afford two weeks just to get attuned to one instance. Whether or not that's a good thing for the game at large remains to be seen.