I really like the limited attempts model on Professor Putricide. For those who are unaware, Putricide is the last boss in the Plague Wing of Icecrown Citadel. You get 10 attempts on Putricide each week, after which you cannot engage him in combat any more.
I like the 10-attempt limit. It's a good number, large enough so that it gives you room to maneuver, but small enough that it really restricts you and causes a good stratification among the guilds. The 50-attempt limit in Trial of the Grand Crusader was way too many. Many weeks, we never even used up all 50. In contrast, only 10 attempts makes it feel like you need to make each attempt count, while still giving you time to talk things over.
(We haven't beaten Putricide yet. We've pushed into Phase 3 a couple times, and have 5 attempts left for our next raid day.)
I'm not sure how the rest of ICC's raid model will work. From what I understand, the attempt limit will increase, but each end boss will share attempt counts. I.e. Maybe next week you will get 15 attempts to defeat both Putricide and Blood Queen Lana'thel. The number of attempts will keep increasing. As well, eventually the faction leaders will provide a raid buff to help guilds complete ICC.
Personally, I think that seems a little bit complicated. I think a better model would be to have attempts on the bosses be independent of each other. Then, the number of attempts should remain at 10 always. For example, you get 10 attempts at Putricide, and then 10 attempts at Blood Queen Lana'thel. Only the faction leader buff will "nerf" the instance as time progresses.
I think this model would be easier to understand, and easier to make decisions about. You don't need to juggle number of attempts and bosses. Additionally, the 10 attempts provides a real limit for all guilds, and will produce the nice stratification that the Putricide limit has created.