Stackoverflow obviously raised the bar for Q&A type forums. Whilst that format might not be suitable for all use-cases, so many sites seem to be implementing the old web 1.0 forum - e.g http://mailwatch.co.uk/forum/
I'm guessing many of these are self-hosted too. What opportunity do you think think there is for a updated, threaded, hosted forum service is? What kind of features would you want?
I think there might be lots of things to rethink on the UX side now that we have so many new frameworks available.
If you aim to build a 100% hosted service, I feel like suggesting to let the client keep ownership of the database at least. I wouldn't like to give total control of all the posts on my forum to somebody else.
I'd say simplier & better looking interfaces + open customization features is the path to follow. Just my 2c.