Woof, after your comment I started looking at the ages on the left and now I'm feelin' old.

What makes a forum professional? One thing is the signal to noise ratio. Unprofessional forums have a bunch of LOL and great post type replies, off topic threads, personal attacks, duplicate threads, etc. S/N is a combination of strong moderation and good content.

Does a good design make a forum professional? No, but it can make one look unprofessional. This forum is one of the best that I've seen.

I personally think that images and colors in .sigs can detract and distract. (sorry Lord Kalthorn )