Some anecdotes from running multiple forum sites over the years:
Most forums end up using flat replies because normal people don't understand threaded replies. A friend at YouTube recently said that their research backed up that anecdotal assumption.
A couple of the major forum vendors tried offering hosted solutions and it wasn't sustainable as a business model for them; they both went back to simply licensing the forum software itself.
Forum communities are like any other social system: network effects and "where your friends are" are what keep you from leaving. Most "power user" features go unused by the majority. Stats like only 3% in one community use the friends list, only 10% in another use the profiles, etc. It's never about the feature set, it's all about the community.
Existing forum software is basically a black box. It's hard to integrate with and expand. It's hands-off for the admin until they realize they want a site that's more than just a forum.