The Forge tackles an interesting question: are fantasy sports MMOs? Some of the more traditional sports fans would probably deny it, but Matt echoes a great case made by Charles Hudson: they feature repetitive gameplay, a combination of luck, skill, and strategy, and there is the obvious social element. I don't know if I would call fantasy sports "virtual worlds" (so this is likely the last you'll see of them on Massively for the foreseeable future), but it's definitely true that both fantasy sports and MMOs share the same type of social gameplay that's made them both so popular.
Now, Matt says they also have a story, and to a certain extent, he's right. The real allure of sports is knowing all the stories behind it-- that Brett Favre's father passed away right before he played, or that the star hockey player is leaving right after the game ends to go see the birth of his baby. Behind all the stats, sports is one big storytelling session, and Sportscenter is the biggest daily soap opera of all time.
But I don't think it's the story element that pulls in fantasy sports players-- it's the competitive element. The numbers all break down, and at the end of the year, you have a clear ranking that says who's better than who at predicting sports outcomes, and by how much. There is a certain amount of that competitive draw in PvP, but MMOs, I think, are far too distant from the actual numbers to make a sports fan happy. They may be interested in story, but at the end of the day, the game comes down to runs, strikes, and outs, and I think that's the way they like it.