Is a forums design one of the main factors of success?

Iwould be more inclined to visit a forum that is not too clumpsy, and would be happy to see a forum having light themes, that are not dark and scary. Though it is not uncommon to see blu-colored themes, I still like our adminforums.org which is too straightforward and easy to navigate.

In my opinion it is one of the main factors.I always register on a forum that has good design.Because it makes the impression of the forum.

But the community itself should make a more important impression.

I believe design is the second most important thing, right after content.
You must have good and unique content and a community that delivers it, in order to attract you user's attention. When you get their attention, it's important to present them a product in nice packing - in this case - a nice looking design.
Although most people (unless we're talking about designers) don't really care about the design - they see how other sites look. And most sites that tend to do serious business follow the design trends - so eventually, users will notice that your website doesn't look as good as the competition and they go to them.

So, bottom line - good design helps a lot

May be! it also depend on theme, members and cool moderator like AF
they always appreciate with all the members.

The theme is what people see as soon as they go on the site, if it looks bad, I probably wouldn't stick around.

I think the design is just as important as the behind the scenes code. Each has its own purpose and if one isnt doing their job the entire site/forum will suffer. It doesnt matter if it is client-side or server-side the design is very important.

In my opinion, yes. I wouldn't like to go to forum where design is crappy and hard to understand.