I have seen people form strong attachments of responsibility to groups while playing World of Warcraft. A friend of mine will frequently beg off social engagements because of scheduled in-game events to which he needs to contribute. Something about the 3-D avatars seems to do the job.

In fact, I'd start even simpler. We need profiles. I'm not saying we turn LessWrong into Facebook, but our brains are wired to track humans, and specifically to respond to faces. I think if we simply had a photo of each LessWrong member appearing next to their submissions (as Emile has also suggested), it would help to excite those faculties. In addition, if each member had a place to write about themselves for a line or two, or link to a personal website, that might help.