For me, it's the ability to meet all sorts of people from all over the world, get to know them in some small way, and share with them the enjoyment of this game I am having a BLAST playing.
Next would have to be the sense of accomplishment I get after finally figuring out how to beat the quest that has killed me time and time again. Yeah, I die. Dying to me is a learning experience. The thing I try to take with me from each death is "Why did I die?" and "How can I avoid this in the next try?".
After that, it's just the sheer fun and excitement of coming face to face with dragons, demons, and all the baddies we love, and beating them in the face of nearly insurmountable odds. I'll remember the raid(s) that very nearly failed more than the one that went so smoothly, it was just another completion.