Based on previous attempts at gathering feedback, odds are that nobody will respond to this thread, but I'm asking anyway.
We're currently using Vanilla for our forum. While we love Vanilla in theory (super lightweight forum, add the functionality you want), we're considering moving to a different forum for a few reasons. (Extensions are great, but there is certain functionality like email notifications and syntax highlighting that would be a little easier if it were built in.)
Our current thoughts are: SMF
(used by MODx and many others) and PunBB
(used by MooTools, Freshbooks, and etc.). SMF is clearly powerful, but it's just SO beastly. PunBB is streamlined and gorgeous in its execution, but lacks a few things that SMF does offer (like sub-forums).
If anybody would like to chime in and influence our decisions, feel free to comment