This is a pretty old post, and it's a question that's been asked before, but it's still an interesting one: what's the hardest role in a raid? Ghostcrawler says he doesn't find one of the three roles (tanking, healing, and DPS) to be "hands down harder" than the others, although he does call tanking and healing "probably more stressful."
As someone who's done all three in raids a fair bit, I feel pretty confident saying DPSing is hands-down easier than the other two roles. Of course, it varies fight-by-fight, but typically the most that's asked of DPS is "don't stand in the fire" and sometimes "switch DPS to X." It does take some research and work to get the optimal rotation down, and some classes (like Death Knights) have it harder than others (cough Ret Pallies), but the raid just doesn't rely on your split-second reflexes the way it does for tanks and healers. Heck, last night I caused a wipe by just saying the wrong thing while tanking.