I don't have a monopoly on the opinion that the Burning Crusade changed World of Warcraft for the worse, by concentrating on a very small percentage of raiders. NewBreed of Work Avoidance Strategies did a wonderful job explaining the problem in this long post.
Most relevant quote in my view: "The fact that now if a person drops connection, doesn’t perform 1 task perfectly, or goes AFK, it wipes the raid. It doesn’t just hurt them, but WIPES them. This is not game design that I wish to participate in."
The thing is that before TBC there existed a strange beast, you could call him the "casual raider", and I certainly was one of them. Every guild had really hardcore raiders, but most guilds didn't have enough of them to fill every last raid spot, and there was room for the casual raiders. As a casual raider you didn't perform quite as well as the hardcore, didn't participate in every raid, spent less time on preparation. But you were still needed in the raid, and able to contribute in a positive way. TBC removed this possibility for casual raiders to do a positive contribution to a raid. Everyone has to play at 100%, and a moment of inattention, or a small technical problem, will not only kill you but wipe the whole raid as well. That is not the kind of responsability a casual raider wants to shoulder.
If you are a hardcore purist, you might be happy that the "slackers" are gone from the raids. The quality of the remaining raiders now is certainly higher than the average quality of raiders in Molten Core. But as people themselves rarely change, that increase in quality was achieved by a decrease in quantity. What I *hoped* TBC would do was split a large raid into two smaller raids. But what TBC *really* did was transform one large raid into one smaller raid, and slam the door in the face of the remainder. I used to be *in*, now I'm *out*, and that isn't totally by choice. I just happened to be in the less dedicated half of the raiding population, and this part now *isn't* part of the raiding population any more.
The percentage of the total player population that is raiding now is smaller than it was last year. I can't say exactly how high that percentage is, but as a percentage of subscribers (not as a percentage of people online, because measuring people online counts hardcore players more than casual ones) it is in the single digits. The game that went up to 8.5 million subscribers by being "for the rest of us" is now more and more turning into a game for the elite, which is why we don't see any new subscription records any more. World of Warcraft has stopped growing, and that is because it lost sight of their customer base.