I've only recently discovered SilverStripe and just to say a big heartfelt thanks to the team for producing such a great functioning CMS - it's a totally killer app.
Tim asked me to post my thoughts re the forum module so here it goes.
Big thing I guess from current/past forum experience would be to add a field for a user to list hobbies or just some general bumpf on themselves to their profile.
...and leading on from there a way to search/list all members (the protocol in most forums regarding this is that the members list can be switched on/off by admin)
this would be a core feature I believe in order to give some personality to the online community - it means for example as a hobbiest or community forum members can detail their particular specialities/skillset/collections etc in their profile the public view of which is linked on their avatar in any threads posted. Maybe also a way for a member to tag themselves in their profile to so that it can be a search option in the forums advanced search feature?
The ability within viewing a member profile to list all posts by that member.
Second biggy is a PM facility, this gives users a way to communicate privately to other members, another community building tool that most of the popular forums would have as part of their feature set. Again it's availability should be set by Admin.
and building on the PM feature or as a separate mod, the ability to subscribe to threads that you are either posting to or interested in tracking would be cool.
Big one re usability that I noticed when I had a scoot around it last night is the following. Once you click into a thread there is no way to get back up to the forum, to jump to other forums yes but not to just go up a level apart from the back button, a clickable hierarchic breadcrumb trail would solve this at both the top of the thread and at the bottom.
...and as mentioned previously the caching problem when editing your post is a biggy.
That's my 2 cents at the mo' and please forgive me if some of the above already exists and I've somehow missed it - keep up the great work guys!