There is a common misconception that people like to play MMOs because people like to interact with other people when they play games. It's true that people like to play MMOs; World of Warcraft hit 11 million subscribers not too long ago. It's also true that people can find playing games with others extremely fulfilling -- my own experience with tabletop rpgs, Left 4 Dead, and even running instances with friends in World of Warcraft has taught me that. However, many players, myself included, often enjoy playing by themselves.
People play MMOs for the recognizition by other players, not for the interaction with them. City of Heroes' badge system and, more recently,WoW's achievement system, give nods to this style of play. Vanity mounts, titles, and sexy, glowing gear all make sure everyone around you knows how awesome you are.
There are, of course, exceptions. The first are large raiding guilds and the second are small groups like my guild: friends organized social-network style where every member of the group knows at least one other person face-to-face. These kinds of groups, however, have something that pick up groups do not: they're not full of strangers.
Pick up groups are terrible 90% of the time. I don't care if you're playing WoW, Team Fortress 2, or tabletop d&d. If you're playing with a bunch of people whom you havn't vetted then you have a high chance of ending up with someone who just doesn't mesh with your style of play -- or sometimes with any style of play. In the real world you would not go into business with four random people off the street. You wouldn't plan a roadtrip by posting a LFG notice on the local bullitin board (ok maybe you would, but you would probably be desperate and would expect some issues along the way; there's a reason hitchiking is illegal).
People form social groups for a reason: to find like minded people easily and to keep people you can't work with at arms length. While mainstream mmos are getting better, many games still throw you into a world and expect you to commit to partying with strangers. Noone wants to do this. Occasionally you get really desperate and either go looking for someone else to help you or answer some msg or plea to help. This usually results from the same kind of desparation that makes people think it's ok to ride in the same car with a stranger for 8 hours, and usually has about the same results.
Enter Warhammer: Age of Reckoning's Public Quests. In brief, these are quests that you can participate in simply by being in the right place. You don't need to join a group, you don't need to start an instance. All you have to do is show up and start killing guys.
These quests have multiple stages. The first stage is almost always soloable by someone of average level for the zone. The second stage is sometimes soloable but is often a 2 person gig. The third stage requires multiple people -- anywhere from 2 up depending on the difficulty and the scale.
Participating in these as a solo artist can eventually give you access to some nice reward gear. Being involved when the third leg of the quest is completed gets you in a drawing for some pretty swank gear.
The result of this is that solo players come to the first leg of the quest to defeat it. If there are a few solo players there together, they might stick around for the second leg. As happened with me twice, they might start hanging close to one another -- a tank tanking, a dps staying farther away and picking things off. If they succeed at closing out the third leg they might stick around for the last section, other players might even come in an join for the chance at the loot roll.
In other words, it encourages players to get to know each others play styles without putting up a requirement to party that gates so many single players. This is not a perfect solution, but it does provide not only something for solo players to do without committing time to strangers, it also provides a foundation for building relationships organically, the way real people do.