There are only 25 slots in a raid, but now with the DK, there are 30 class/specs combinations. They want all the classes and builds to be viable, but now there's no room. The question, said Street, is now: what can we do to make sure players have more flexibility with WHO they bring?
Before, in a raid, Buffs felt out of control. It was complex, it was inconsistent, there were stacking problems - Curse of Shadows benefited Warlocks, so you would get more benefit from bringing another Warlock instead of a Mage. Lots of different specs now are going to bring something like Curse of Elements, and with the prevalence of raid-wide buffs, even if the Mage and Warlock are in different groups they'll get the benefit.
Bloodlust was a "great spell idea," but got out of control and scaled too well. The developers also didn't want to run into situations where raiders would say, "Well, Jimmy can't come, so we don't have Windfury ... let's cancel the raid." They don't want one buff to be a make-or-break deal.
Most buffs now benefit whole raid, and there are multiple ways to get same effect (that don't stack). The philosophy they're trying to bring is that you want raiders to bring the player, not the class. They wanted to make sure not only did everyone bring something that could buff, but that everyone COULD do DPS - where player skill matters more in the damage you do, instead of class design.
A word that Street mentioned was used a lot in the class design team was "Burden." They were concerned that so many of the consumables in the game were a burden - they like like potions, and they like drums, but they don't want people to have to farm 200 of them to raid. They don't want the game to be about consumables, but about "Do you know what you're doing?" Gear and skill should win out over buffs and potions.
If you want to raid, just bring good players.
There's more shared gear. The change to Spell Power lets them consolidate, they no longer need a Healer ring AND a DPS ring. It also lets healers do DPS once in a while. People do PUG 5mans, daily quests, and they don't want to have to go respec.
With four viable main tank classes, they need to overlap their niches. They don't want raids saying "let's set the DK out for this fight, bring in the Paladin for the next one, etc." They want raids to have two MT classes, and then go do what they want to do.
Tank threat is much higher than it used to be. Encounters are now about "Can you keep people alive, can you move around the room?" and less about playing the threat game. Threat is important, but it shouldn't be "Oh, some DPS pulled, we wiped."
They are working on a system where you can Dual-Spec. They want you to be literally able to swap specs in the middle of raid, and not have to run back into town: they want it to be easy and not complicated. You won't be able to switch specs in the middle of an Arena fight, but if you like to tank on weekends and PvP on weekdays, that'll help you out.