Some of the problems with the level of synergy characters have in DnD being brought to WoW:
1.) People play classes they enjoy. Each of the ten classes behave very differently, and in a game like WoW, where certain composition is expected in 10 or 25 man raids (2 tanks, 2 healers, 6 DPS on average for 10s, 2-3 tanks, 5-7 healers, and 15-18 DPS for 25s), the only rule for 4e is that you want a minimum of one striker, one controller, one leader, and one defender. Any of the classes in DnD can double up effectively, but in say, five man instances, going with a pair of tanks or healers can be hindering. The same is true of going with four healers or four tanks in a ten. Enrage timers have made fights in WoW exceedingly precise in what they require to be completed. Not only that, but there are only four healing classes and four tanking classes (two classes are also capable of being tanks or healers).
2.) Arena. This is the constant thorn in the side of many PvErs and PvPers alike (especially BG enthusiasts). Arena has greatly modified PvE tactics over time because of certain abilities or talents that were causing immense problems in PvP, but were just fine in PvE. The Fan of Knives nerf for Combat (removing the interrupt effect from Throwing Specialization) was one example of this, FoK interrupt was required for a few fights in Ulduar, which was the highest raid at the time, but the ability for a single rogue to interrupt five people all at once was just too much.
3.) Fight balancing. Fight balancing WILL be modified over time to favor certain combinations of synergistic characters, because their potential for DPS or what have you would otherwise completely trivialize an encounter. This in turn would make it difficult for any other group to succeed, because their maximum potential is very limited. But what do I know, I run with four elemental shaman (currently one of the lowest max potential DPS, if not the lowest) and no rogues. :u
Anyways. I look forward to the Cata changes myself, it'll be nice not having ArPen as a stat anymore. But then, it was a stat that played favorites, and frost/UH DKs, ret paladins, enh shaman, and 2/3rds of hunters were not on that list.