At AGDC 2007, Damion Schubert gave a really excellent speech on MMO game design and player retention (which you need to go read if you haven't seen it before). Part of the speech talked about the "exit points" of an online game (his list is to the right), which are points or events in a play cycle at which players will stop playing. Now, the list he gives is a little fuzzy. I'd argue that some of those are exit points, and some of those are just convenient times to stop paying for the game, which won't happen unless the player has a reason to stop. Lets ignore the first two points for that reason. However, the last eight are all examples of reasons that a player might leave. A game designer's job is to reduce the likelihood that a player will want to leave at any given point.
It's important to note that those are hypothetical issues for a single player. Obviously, you and I probably aren't going to have problems with using our mom's credit card (I have my own credit card, thanks). People's reasons for quitting will vary a lot depending on their personal situation and preferences. What I think is really interesting, however, is that these points can be divided into three basic categories that could be applied to anyone's reasons for leaving: gameplay issues, social issues, and personal issues. For example, guild drama (or discovering that your hot elf friend is actually a man) are examples of social issues that might cause a player to quit, while hitting a wall in your advancement is a gameplay issue. Discovering girls, feeling like your gameplay is meaningless, or being unable to afford to continue playing are all personal issues.
Of these, gameplay issues are by far the easiest for developers to identify and fix. They're the ones that they can exert the most control over, and they're the ones that players will be most honest about when reporting reasons for quitting -- no one is going to admit that they're quitting because they're uncomfortable about accidentally flirting with a guy. In fact, developers seem to focus on identifying and fixing gameplay issues almost exclusively (which is what they do every time they patch the game), so that's not a problem.
However, the personal issues and social issues are very real reasons for why people quit games, and a successful company is going to be able to recognize that and try to curb them. Personal issues in particular are hard, because there are many of them that you can't and shouldn't be trying to prevent. Getting players to continue paying when they literally don't have the money for a $15 per month subscription is a terribly irresponsible idea, as is trying to prevent Billy from noticing girls -- stunting his growth in that area will probably cause him to become a bitter, hateful forum troll. Providing alternate subscription models for people who don't have credit cards, on the other hand, is a great example of solving a personal issue in a positive way.
Still, it's with the social issues that I think companies could really gain a lot of ground in the retention area. Social issues are harder to fix than gameplay issues, but much easier (and better) to try to fix than personal issues. While I don't have any numbers on this, I can say anecdotally that I've heard of many, many players quitting due to guild drama, a lack of people to play with, being unable to play with their friends (that one actually skirts the edge of the gameplay category), quitting due to jerks in the community, and the like. All of these issues can be influenced through good game design, if not fixed outright.
Some examples of ways that you could address social issues are by providing better guild tools (both for the recruitment of players and for organizational tasks), offering players a way to self-moderate within their community (which is rife with abuse potential but possible to design well), and providing better mechanisms to match up players who are similar in skill level and temperament to one another (as opposed to the totally random tossed-together mishmash we have now). Successfully identifying the problematic social issues and fixable personal issues in a given game are the first step towards eliminating them, and the best way to do that is to talk to the people who play it. If your endgame is designed in such a way that players frequently burn out, or if guilds often collapse due to drama, developers need to know that and understand why it's happening.
I think that the industry has spent the last ten years figuring out the gameplay part of the equation. Blizzard came in and showed us that gameplay can be polished to a brilliant sheen that makes millions of people want to play. Now, everyone is focused on that: polish, polish, polish your gameplay. Despite still being extremely important, that's old news. Everyone knows that now, and everyone tries to focus on it. There's still plenty to do in that area, but I think that the area where real innovation could be made in the industry right now are the other two aspects of retention: social and personal issues.
I'm starting to think that the next really big game isn't going to make huge strides on MMO gameplay, but it's going to be the one that comes in and shows us all how terribly awful the social and organizational aspects of our MMOGs are. They'll do it by designing that area in a way that's simply, elegantly, and beautifully better, and it's going to seem completely obvious in hindsight. Players won't be quitting their game because of the social and personal issues that plague games like World of Warcraft, and people will want to play it because it has fun gameplay and good social tools. After all, that's exactly what Blizzard did with gameplay issues, and look where they are now -- retaining 10 million customers per month with some serious player turnover thrown in. Fixing social issues and some personal issues could cut that churn significantly, which is what I'd wager many WoW players burn out on.
After all, gameplay is nice, but these are primarily social games, right? Shouldn't we be focusing on successfully doing the social part of the equation as much as, or more than, the game part?