Well, I've contributed my worth, opinions, and suggestions at the "Improving the Forums" thread in the Feedback and Suggestions subforum.
Though what makes a good forum should follow these standard base rules I believe. (Accumulated through forum hopping over the past 4 years.)
1. Active admins and moderators who do a good job in keeping the forums alive and controlled. Forums flow smoothly with new material, and if problems arise, they can be resolved.
2. An active memberlist. As long as a forum has more and more people ineracting, that's a sign of a healthly and godd forum. Inversely, a large memberlist does not mean huge member interaction. The two are separate and interaction is more important of the two.
3. Easy forum user interface. That means that it the forum skin is not stupid/non-user friendly, and that it is easy to navigate through the forums, e.g. quick reply function, search-bar, memberlist, etc.
4. Similar to point 3, forum organization. No repeated, stupid, useless, misplaced threads. Orderly subforums and sbfrm categories. If a forum strives towards that, it makes the forum a cleaner and more efficient place for members to browse in and interact.
5. A basic set of rules for all members to follow. Nuff said.
6. If I can think of anything else, it'll go here.