The user interface at http://www.xenforo.com is stunning, intuitive and very easy to use.  I like Simple Press too but am a bit dismayed with the clunkiness and bulk of the user interface.  If you could make it more streamlined and easy on the eyes without all the clutter etc. like Xenforo has, you would have a killer forum.  

WordPress is dying for a fully integrated forum solution and it looks like BBpress is dead so maybe Simple Press can take that role.