As we've come to expect him to, Ghostcrawler responds with a fairly in-depth answer. He covers the difficulty of balancing two or more specs in a class that fulfill the same role and how this experience in Wrath of the Lich King helped guide Blizzard in its systems overhaul for Cataclysm.
I'm going to quote the entire reply in full because it's got implications for the game well beyond Wrath.
There are two very interesting aspects to this answer from my perspective. The first is the idea that player behavior creates feedback that affects design during as well as after an expansion. Not only were these four specs affected by player behavior (i.e., it was player choice that caused the development team to fear going too high with these specs because it had witnessed players' feeling "forced" to switch out of BM to survival when that spec was buffed), but the design of Cataclysm going forward is directly addressing this tendency by giving the developers more knobs to turn, so to speak.
Secondly, the idea that players are willing to completely relearn a class or spec between expansions with much greater tolerance than they are during an expansion has implications for how and when designers can adjust a class or spec that isn't matching up. In fact, this reluctance creates a situation in which, as a designer, you have to aim for gradual increases until you're given the freedom of redesign offered by an expansion patch.
What makes Cataclysm so interesting there is that it is an expansion designed to give more tools for such small corrections during its own life cycle, in effect taking the lessons of The Burning Crusade and Wrath and incorporating them into the systems of the game itself. Since we know small corrections will have to be made and that very often those small corrections will have unforeseen consequences (like making one spec overtake another), one of the goals of Cataclysm's design becomes incorporating more hooks for the design to be adjusted around, to prevent players from feeling forced to switch specs to be competitive.
The idea of designing the game to alleviate that aspect, to make it less likely that players will be frustrated by waking up and being forced to relearn their class, is definitely a pretty large shift for the game. If it works, it could create a World of Warcraft unlike anything we've ever seen in the six years of its history -- one that has learned from itself.
World of Warcraft: Cataclysm will destroy Azeroth as we know it; nothing will be the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion (available Dec. 7, 2010), from brand new races to revamped quests and zones. Visit our Cataclysm news category for the most recent posts having to do with the Cataclysm expansion.