Anybody trust Blizzard's testers?

There is no "balance team." WoW has a systems design team composed of game designers. Some of these designers have many years of industry experience while others were recruited because of their expert theorycrafting or in-game skill (and often both). The systems design team is responsible for everything from combat mechanic, achievement, and UI design to tradeskill, class and item design. It is one of the strongest design teams in the entire industry, which I feel like I can say, because several of us have been around in this industry for some time.

In addition to the kind of tests we can do at our desk, we also rely on a large and experienced QA team to find bugs and test implementation. We also have a terrific community team that helps to funnel us information from these and other forums. We also have a lot of contacts among expert players and theorycrafters, in both PvP and cutting-edge raiding, out there who serve as a reality check for what we do internally. We talk to players all the time, through email, through the game, or through forums. All of these are pieces of the process and I would be hard-pressed to say which is the most valuable or has the most influence.

All told, it is a very robust process and I think that WoW is so fun and has such a large and dedicated audience speaks for itself. I won't claim that we never make mistakes. Sometimes players go in a different direction than we predicted. Sometimes they find powerful combinatorial effects that we didn't know existed. Sometimes they just play better than we anticipated. Sometimes they use exploits. WoW is a very large game with a very large community. Stuff is just going to come up. It happens in any kind of game, and especially in an MMO.