One feature long missing, and oft requested in VB, is the ability for users to view and post messages to the forum direct from email. The lack of this feature is what keeps hundreds of YahooGroup admins from moving to a more robust forum. I use an add-in to support this functionality in VB 3.x but it is not available in 4.0.
If this is a feature supported by xenForo, I will by buying a license today!!