I like how Blizzard is willing to keep trying new things and uses WoW players as guinea pigs.
I've said (once, in a very obscure post) that other games take tried-and-true systems from successful games and end up stagnating because they refuse to be truly innovative. This is because innovation carries a huge risk of failure and companies aren't likely to invest in a business model that doesn't have a good track record.
Blizzard, on the other hand, doesn't put a choke hold on WoW's game mechanics just because they might screw up whatever it is people like about the game. Instead, they add new things and test stuff out and keep pushing forward and learn from their mistakes.
Yeah, there are systems in WoW that players don't like. But there are plenty of things Blizz has put in that we do, and they at least make an effort at fixing what we hate about the things we hate.
So while other companies are talking about how WoW is popular because gamers like to kill each other in world combat (*gag*), Blizzard is testing all their innovative ideas (battlegrounds, arenas, glyphs, mastery) on their target audience and getting real feedback as to what systems work and what could work if they tweaked it and what doesn't work at all.
Blizzard's willingness to experiment and push forward is what I believe will keep them ahead in the long run.