Having played WoW for the last six years, and Everquest for nearly 6 years before it, I'd say the primary reason I play MMO's is the sense of constant progression and the people I play with, with emphasis on the latter.

With many games, I might spend 30+ hours developing and honing my character, but eventually it comes to and end. With MMO's that development and progression need not come to and end in the foreseeable future.

I am very aware of the repetitive nature of the genre, and the somewhat shallow nature of said progression. However, it is the constant challenge to improve, and assist those I've come to enjoy playing with, that keeps me around.

Personally, however, I like the penalties associated with the genre. Even MMO's seem loathe to impose too harsh a penalty for failure these days. In WoW, you die, you lose a few gold to repairs. A few years ago, in Everquest, you died, you risked lost experience, and even level loss, if you weren't careful. However, that such a system also made success feel all the more significant as a result. Too many games, and game developers, seem afraid to penalize their players these days, in my opinion. Risk is essential to rewards. Without penalty, those rewards, when achieved, seem less significant to me. If anything, I'd prefer to see the current generation of MMO's be more difficult and punishing than they currently are. I'd rather see them drive their players to improve themselves, rather than cater to them despite their shortcomings.

I completely realize, however, that my personal preference puts me in the minority, and is not in line with the easy rewards that the majority of MMO gamers desire.

However, ultimately what keeps me playing are the people I play with. I started playing WoW as a way to spend time with my now-wife when we lived on the other side of the country from each other. What keeps me playing are the friends we've met, and to a lesser extent, the challenges that are left to conquer. Although, I must admit, that the limit of such challenge has recently become much more apparent to me, and that I've been quite tempted to abandon the MMO genre completely, as of late.