I think some of the other players have answered this the way I would. The idea is not that you can bring 25 prot warriors because they are all excellent players and expect to clear Naxx. I exaggerate, but you get the point. The idea is that you have enough coverage over "critical" areas with fewer players so that you have enough free spots to bring who you want. I put critical in quotes, because it varies depending on groups. Some players think Heroism is mandatory. It probably isn't for Naxx but it probably was for Sunwell. Some kind of poison dispel is probably mandatory for Naxx. Innervate isn't mandatory. Battle rez is useful while learning the encounters. And so on.

Having two priests is currently mandatory for heroic Naxx to do one encounter (that gates some others). We knew we were pushing the envelope with what we could expect of of raids with that design. I think most 25-player groups have 2 priests, but I'm sure not all of them do. If we had asked you to have say 2 Survival hunters, that would clearly be over the line. If we had asked you to have a death knight MT for Saph, that would have gone against the design. Perhaps we should have left up one crystal so you only needed one priest.

I think expecting one of each class isn't too much to ask for a 25-player group, provided we aren't asking for a particular spec or a particular role (like a druid tank or a paladin healer). But this is the kind of thing we're going to have to explore a little more to see how it feels.

Ulduar is going to be more difficult than Naxx, and we expect to see a lot more concerns or complaints about specific classes, specs or buffs being mandatory. We're not designing the encounters that way, but with tougher tuning, I wouldn't be surprised to see more given how much we've seen about relatively easy raiding content. We will deal with those problems if and when they arise.