Icecrown Citadel and killing Arthas' men. And having been a roleplayer in one medium or another for longer than Robert Pattinson's been alive, I tend to do all this in character. (I have forsaken vampires, though, don't worry.)
However, we can't always play in a perfect vacuum, with friends or with functional equipment. Real life happens. I currently live on a mountain. Awesome views, but really bad internet. My internet connection drops like it's hot. I love playing with friends, but time is limited: I work two jobs, train the Dark Puppy and am maintaining a garden, a marriage and a household. And I try to stay fit in the meantime. Ultimately, this all means time, friends and equipment can all be in short supply. I keep up pretty well in terms of gear and skill, but I just can't be around much.
While WoW is probably the most friendly MMO out there for the casual or time-restricted player, it's still a pretty big bummer to always be the guy getting dragged along in a raid. No one wants to be the deficient player. And more than a few times, it's left my mouse pointer hovering over the "cancel" button.
But crafting and professions have saved me, every time.