I am a long time fan of MMA but relatively new to forums, so I thought I'd chime in here. I was aware of sherdog before I joined MMAforum but I only signed up here. Why? As was commented in the thread, sherdog is massive - which has its pros and cons. Personally, I look forward to being able to contribute to the development of a new forum that will have a more personal feeling to it then Sherdog.
I also agree that in addition to getting inspiration from other forums that have been successful for ideas, we will likely come up with some great ideas on our own that do not exist on Sherdog. As members, I think we all share a common goal of creating a great community of MMA fans here - I know I do.