I've played World of Warcraft for the entire history of the game, since about a month after launch (my wife actually played in beta, and she's the one who got me into WoW in the first place) and I've raided for pretty much the entire time - I took a few months off after The Burning Crusade dropped, and had to catch up in BC raids. Since that time, though, I've raided - I was in my server's most progressed guild in Wrath, switched servers but ended up in the same situation in Cataclysm, and have settled down to a still well progressed but less aggressive heroic raid in Mists of Pandaria, cruising at 10/14H and working on Thok. We have one pally, so Thok's a bit of a gigantic cinderblock wall, but we're still plugging away.
Being that I've been raiding so long, I sometimes see patterns. There's one I saw in BC, and repeated in Wrath and Cataclysm - the end of expansion lull. Once we get into the last tier of content, there's a surge of interest and everyone leaps to get in there and work on it... and that lasts a couple of months. After that, however, interest starts to wane. Players get burned out, stop playing, need to be replaced. Each player who needs to be replaced causes tension as the guild slows down due to the losses. Recruitment means bringing in people with less gear, less experience, and even if you manage to get a player with both the gear and the experience, it doesn't always mean they know how you do things. I was once recruited, after my Horde guild had killed all of Heroic Dragon Soul, by an Alliance guild that was on Spine. I took the jump because I wanted to play Alliance again - and even though I was geared as well or better than they were, I still had to relearn the fights based on their strats, and make suggestions based on my own experience that meant delays as they learned these new ideas.
This can lead to a feedback loop - players burn out, leave, this stresses the guild, more players get burned out. It's always present in raiding - churn is inevitable, recruitment must be continuous - but the promise of future content to come creates a counter pressure. You don't just raid to see the current content, you do it to be ready to get into the guts of the new stuff when it drops. But when you get into the last tier of raiding, there is no new content to keep you interested. And so, when that last raid tier takes months and months - sometimes, as in the case of ICC in Wrath, over a year - it becomes very difficult to keep guilds focused on progressing through it. Talking on twitter about all this after reading multiple posts on the issue, I started thinking about how it works out.