I’m an old ASP diehard coder that’s finally moving away from the dark side and into the world of PHP, AJAX, Web 2.0 and the other new buzzwords. Extensibility is my main reason for shifting. I really like the idea of making as much of the forum functionality as plugable as possible. In my mind it accomplishes two things:

1. Allows for more CMS-like functionality for those of us that use WP more as an application platform than a blog. 
2. Every time one of us "dumb ass users" asks for a stupid feature (like supporting integration with every other plugin we have installed) and you dont want to mess with it, you can say "Sure you can do that - just write a hook into XYZ function and then pass it off to your ABC plugin"

I also like the private messaging plan. As part of that can I recommend an option to send admin messages as either pm, email, or both? Might be a nice touch for user subscription options as well. Whatever you do with PMs and plugability overall it would be a bonus to have at minimum the capability to plug into the PM send method so we can make any part of WP drop a note in the admin’s in-box.

Yet another nice feature might be term/taxonomy support on a per-topic basis, combined with a "related posts" method that could be optioned into The Loop to  match forum topics to posts. Similar to the way NGG gallery does this for related images. I like the already existing "Discuss this post in the forums" feature, and for most cases I imagine a one to one relatiosnhip like this is enough. But by using terms it would create a many to many relationship, opening up all kinds of possibilities. For example, on my site I use tags more as a glossary, and posts are more like articles or wiki entries. So instead of having just the one dedicated forum thread linked on the "Saturn" article I’d be able to display something like the 5 most recent related threads based on weighted tags. I think that would go a long ways to extend the penetration of forums into the more static content.

Oh well, at this point I’m just brainstorming. Hope some of it you find useful. :) 

Keep up the awesome work, and thanks for letting us benefit from your labors 

I’m sure no matter what you come up with it will add to the whole package nicely