The obvious answer is that I still play because I still enjoy the game. To be more specific, running dungeons is one of my favorite things to do in World of Warcraft, and Blizzard consistently does an extremely good job with their dungeons. I've also been a member of the same guild since early 2005, so logging into WoW is like attending a gathering of good friends. Some of the people I talk to in the game are people that have been a part of my life since I was a teenager (which may give you some indication of my age, for better or worse).
I will admit, though, that I play significantly less now than I did a few years ago. Considering what we do here at WoW Insider, I'm still extremely interested in the mechanics and evolution of the game, but as I've grown older, I've realized I need to make a distinction between what I do and do not enjoy in video games and need to stop forcing myself to do those things I don't like. I enjoy having goals I can accomplish solo. I enjoy having content I can clear in small groups, like dungeons and the upcoming scenarios. I used to enjoy being a raider, but I lost interest in that during Wrath of the Lich King and don't raid anymore. I didn't enjoy needing to schedule my time around raiding every week. The recent addition of the raid finder gave me the ability to check out the new raid a few times without needing to commit myself to an endless raid schedule, which I appreciate.
I used to be one of those people who made myself miserable and forced myself to log in during stretches of little to no new content (such as the nine-month period since the last content patch), but I've stopped doing that. I still follow the news, PTRs, and beta realms, but I really don't need to log on to live realms just for the sake of adding to my /played. I can take a break and play other games, I can try other MMOs, I can do whatever I want with my free time -- and spending time doing other things adds to my enjoyment of WoW, preventing severe burnout when there isn't a whole lot to do. I still play WoW because I let it happen naturally; I don't force myself to do it. When there is something new and I can accomplish some new goal again, I can go back to WoW full-time without any fuss.
Why do you still play WoW?