What do you think makes a forum professional?

I think that someone already said friendliness is important and I have to agree. I have seen so many argumentative members in other forums destroy the whole life of the forums. Some of those forums even closed down due to flame wars. So, friendliness and appropriate moderation to keep the community growing together are important.

Moderators an admins should also not be on a power trip, many forums have staff members that think they can do anything. Make sure your staff are working in accordance with your site plans and directives.

I also believe you should not restrict the age, maturity is a personal quality, not an age quality.

The design should be good and extremely fast loading, the speed is important to keep members. Also it should look professional, the 1337 gfx the gaming forums have will drive away the mature members.

A lot of immaturity is also displayed in the txt/chat speak which is becoming more common today. Try to clamp down on this type of talk and these kinds of members.

Signatures I feel should be restricted to a few lines (3-4) of 10px text. Images can detract from the forum, especially if the colours are loud and clash with the forum colours. If you do have to allow images then be very strict on the rules and limit the size to very small W x H and small file sizes. Also make appropriate rules as to the content of the signatures. A couple of the biggest forums I am a member of do not allow images in signatures, only as avatars.

Do not allow smutty imagery or talk in the forum, this can very quickly lead to the whole forum dengenerating into a sleaze fest. This will in turn bring immature members to the forum looking for a quick thrill with like minded members.