I suspect I'll end up with a slightly rambling response, but please bear with me...
Forums are my favorite on-line community tool. Its normally harder for things to get "lost" in a forum than in either email or IRC and its a wider audience than standard IM style networks. Unlike a wiki, forums allow for an explicit conversation rather than the hidden conversation via edits.
However that said, there are places where I feel forums could be improved, however, most of the issues come down to effective moderations and admin more than the underlying software.
Features that are very important, and already tend to exists -- quick post/response, find-all-unanswered posts, subscribe to a thread. These three combined allow for "helpers" to stay connected -- they can quickly find the posts that no one else has helped with yet, give a answer/suggestion easily, and then stay plugged in to assist with further refinement.
A current feature that can probably be improved via some tagging method, is the breakup of the forum into subforums. Most forums, this one included, have a number of categories that the admin/moderators have decided partition the set of likely topics. These are often rather arbitrary and often cause no small amount of flamewars over the life of a forum -- many programming forums have "beginner", "intermediate", or "advanced". Often the beginners forum languors unanswered as people get tired of answering the same old question every 5 minutes, and then the entire membership starts to rot as fewer people "graduate" into the non-beginner area. Forums that keep a unified main forum, tend to have a more vibrant community, but harder questions often get lost instead. Some sites have the more advanced topics have seperate names, without calling themselves advanced -- "TDD", "Theory and Design", etc. This seems to work well and people don't seem to disregard the main subforum.... However, perhaps some tagging based system that lets the user set up their own arbitrary subforums and sort things appropriately could be nice.
Finally an integration with a wiki of sorts could be nice -- a lot of programming forums end up with a horrible number of stickies. Forum rules, links to good resources, debugging hints, how to ask a question, cheat sheets, .... the list goes on and one and in many cases the content in these stickies would probably be better served by a wiki style page maintained by the admins/moderators without comments, etc.
Anyways I guess that covers enough for now
My RoR journey
-- thoughts on learning RoR and lessons learned in applying TDD and agile practices.