I recently apped to the top guild on my server, and was surprised at the amount of membership churn they go through.
I spent one week with them, and they lost three people: a rogue (who won the dagger that week from their first kill of Kalecgos), their main tank, and a hunter. But they have a lot of apps, and almost all of their apps are T6 level.
In my previous guild (T5 level) we didn't get very many applications. But we also didn't lose that many people. I would peg our attrition rate at about 1 person every 2-3 weeks (and that would usually be an applicant priest who stopped playing for some reason).
Is this rate of loss (3 members/week) normal for the high end?
we have people that quit playing or quit showing up pretty regularly, but rarely have people leave the guild unless they are just in it for loot or have some personality conflict. a lot of people are busy with school and family and come and go.
edit: we are T5 about to be T6. not quite high end but working on it. server also has about 10 horde guilds in BT/MH so there are plenty of opportunities to raid.
The one guild on our server that's on the cutting edge is going through very high turnover right now. Those of us in the next clump (there's about six of us finishing BT or working on Kalecgos) seem to have a lot less churn.
I think a lot of it has to do with the burnout from people that either never experienced real high end, hard core progression raiding before or that aren't used to it in the slightest after several months of having 'beaten the game'. Putting that much time and stress in has got to be tiring to say the least.