Blizzard's observed in the past that there's often a correlation between the perception of a class as overpowered and the number of people who choose to play it (witness the proliferation of rogues in classic WoW, for example), so it seems fair to say that at least a portion of the player base's class choice is impacted by the conclusion they reach on design decisions. Then again, my own experience in-game -- and the pattern of comment votes here on WoW.com concerning class changes -- leads me to believe that yo-yoing between classes based on which one is doing "best" at any given time is not the overwhelming trend. The Warcraft Census' numbers on class population also seem to be evening out, slowly but surely, from a little bit over 6 months ago (which was itself an improvement over very lopsided numbers in favor of death knights and paladins shortly after Wrath went live). This would seem to suggest that, over the long term, people continue to play the class they like most for reasons that survive design changes. Or is it just that each character represents such a significant time investment that most people don't think it's worth it to switch mains?
I'm sure that arena and PvP as a whole wind up driving a portion of this, but what impact do class nerfs and buffs really have? If your main was ever nerfed, did you wind up playing a different toon, or did it just not matter that much to you? If your main was buffed, was it genuinely more fun to play?