I used to avoid MMOs because they always seemed like a genre that I could get way too into. There were Too many stories about people becoming hopeless addicted and I knew I was susceptible to that kind of addiction, so I just didn't touch them.
Eventually, my curiosity got the best of me and I tried Lord of the Rings Online when it became free to play. At first, it really sucked me in and I was really enjoying it. I even spent some money to open some more of the world up to me. After a few weeks I burned out, mainly because the guy I typically played with changed shifts and playing by myself just wasn't the same.
When The Old Republic came out I thought that the added voice acting and focus on narrative would keep me interested longer. In the beginning it was cool but again, once the shiny new coating wore off, I burned out again.
I'm actually surprised that it's a genre I can't get into. Character building and looting aspects of games grab me more than just about anything else.
Oddly enough, the one aspect that I did get a little addicted to was selling things I crafted or crafting materials on the auction house.