First, I think the forum here is really good from a social point of view; people are really helpful and well-behaved compared to many other forums I've seen. With that said, I have a few forum itches I'd like to scratch, and some random ideas that might be of interest to other people. These are lower on the priority list, but I think they would be totally awesome.
-- When you create a thread, have "email replies" checked by default. Or, slightly fancier, maybe it could remember what you did last time you created a thread and use that as the default.
-- When we use the @userid notation, send a notification to that user. Probably would need to be configurable as an option as some people won't want that kind of email.
-- Make each person's userid/avatar a link to a basic profile page. Since people are constantly changing avatar images, it'd be nice to allow us to provide a little bit of extra information. You'd have to get each user's permission to display this stuff, of course. One problem I have is that it's often not obvious what gender a user is; you can sometimes guess from the name or the image, but this is not 100% by any means. Maybe we could also have links to our blogs, twitter, etc. Eventually the profile pages could show our stats, and that could go together with the long-awaited leaderboard project.
-- Provide a convenient way to find all posts and comments by a particular user. As it is now you can search by userid but this is not super obvious.
-- Allow people to "like" comments. We currently do this informally with "+1" but this could be made easier with a "like" link.
-- Allow certain forum threads to be made "sticky", or maybe have a separate list of "recommended reading" threads somewhere. I think there was an old thread about an HSK success story that we discussed recently; it would be nice to be able to highlight threads like that, and maybe some of the threads that cover some of the frequent questions that keep coming up.
Other people please chime in, especially if you have a different idea...
James