I realised this morning that we've been operating in a cycle that's nearing the end of its second run, as we approach the second expansion.
Conjecture is this: the levelling game is inherently casual friendly, the end game not so much.
So pre-TBC, we could level as we liked, when we liked, and there was very little forced interaction with others (but plenty you could arrange if you wanted). Once we hit 60, our options were few - raid with 19 others (possible casually, but not terribly rewarding without some sort of effort and commitment) or PvP your way up a league table (not very rewarding without effort or commitment. If you slow down, your payback literally decays.)
Then TBC came out, we were all back in the levelling game and, though I've no doubt the main attraction was the new content, I think the fact that we were able to get real satisfaction from playing as and when we liked was what reinvigorated the game.
And then we all hit 70, and suddenly we're getting burned out and bored. I don't think the problem here was lack of content so much - it was that the content that was there required a consistent, concerted effort in order to be satisfying.
What Patches 2.3 and especially 2.4 did, as far as I can see, was reframe the endgame so that you could get a lot of enjoyment out of it without having to raid super-regularly. Between dailies, badges and (the first bit of end game to get this idea) PVP, you do it when you like, with who you like, and progress at your own speed. Which takes the pressure off, even if do you choose to raid a few nights a week.
It's not the only factor. In our own group, I think that smaller (10 man) raids from a larger pool of people probably also keeps pressure down and enjoyment up. And of course, it's currently a great game just in the sheer content available. Wouldn't be the same without that. But I think someone in there has managed to nail, in a way most of us never quite pinned down, how to make casual players feel hardcore without making them feel the pressure. Bodes well, imho, for the entire cycle of WotLK.