There's one type of person in Wow who really get on my nerves. Basically, these are the types of people who actively speak out against trying something if the raid or group doesn't have the 'optimum' makeup.
For example, our guild was at Garr in Molten Core the other night. Garr has 8 adds. The 'normal' strategy is to tank four of the adds and Banish the other four. However, we only had 2 warlocks to banish. We did have 6 or 7 warriors and a strong amount of healing, so the fight was possible with a change of strategy. However, one player spoke up and declared that the fight was 'impossible' without the extra warlocks, and we shouldn't even try.
This timidity is the one thing that really bugs me in WoW. Why not try? The worst that happens is we wipe, and we count that as a lesson learned.
The next week, we had enough warlocks to do the fight normally, and we killed Garr. Thinking about continuing on to Baron Geddon, that same player says that we don't have enough Fire Resistance in the raid to continue. Argh! 
If we tried, wiped, and then came to the conclusion that we needed more Fire Resistance, that would one thing. But to be unwilling to even try?
You see this even in lower instances. People who won't do anything if the group makeup is not exactly correct according to perceived wisdom. Or maybe certain people have the wrong talents (Feral vs Resto druids, Shadow vs Holy priests, etc.).
It's odd, but whenever I've grouped with people from the very elite, cutting-edge guilds, they don't seem to adhere to this 'common wisdom'. Maybe it's because what I'm working on is trivial to them, but they seem to be more willing to try less conventional solutions.
 I do have to say that the player in question is a very good, solid player. I just wish he'd be a bit more willing or enthusiastic about taking risks.