Clerics: Clerics are in high demand, but I consider them to be the donkey class in the game. If they are the first ones to get blamed in a failed battle because they didn't heal their party fast enough.
This part apply to every mmo with dedicated healer. People never want to blame themselves for their mistake so they use their scapegoat (healer) instead. It makes them feel less pathetic at their lack of skills and/or their stupidity. While it does happen that there are some bad healer I'd say their ratio is something like 1 bad healer for every 20 good one. Yet the 20 good ones are blamed upon when something goes wrong. This is pretty much why I stopped playing healer/cleric in games even though support is my favorite role.