As someone who has over 35000 additional lines of customised css in my theme I, maybe strangely, don't think that a forum design is overly important. I've seen many massively popular and active forums that still use the core software default theme with only a few additional widgets usually listing latest posts/threads and some advertising.
I think it all depends on the forum niche. For example, a website about cars is less likely to need a custom design than a general discussion board, role-playing or sports fan board.
At the end of the day, content is always going to be king and determine the success of any discussion board.
Me, I just like pushing my own limits, making something unique and hopefully more practical for my members to post their content on.