We've specifically had a petition *not* to change the forum before:
http://www.webdiplomacy.net/forum.php?
threadID=451106#451164
The forum works well. Game adverts do better for not being separated from other posts- if you had a "game adverts forum", it would do no better than the game listings do anyway- why would you go there.
"Spam/Useless" type threads occupy a couple of slots only on the front page, and are quicker to skip over than to go into a separate forum to avoid.
"Political/Discussion" type threads are more popular for the fact that people can read the intro even when they aren't specifically looking for that kind of thread, so you'll get more contributers than just the hardcore few who are always commenting in them.
A "Questions" forum would hardly be one people frequented, yet when they see a question here, they take the time to help out because they've had no choice but to see the question.
And site news that doesn't deserve a place in the top banner gets plenty of airing through bumping the thread etc. this way too, without having to be "global".
By not having categories, we can probably actually *save* time for a medium sized forum. We also make sure that there is only the one community. Having the 4/5 line intros is better than just a subject line. Finally, it fits into the site better than a phpBB forum would. No to phpBB!
That said, it would be nice to not have to reload the whole page every time you open/close a thread, and some formatting might be quite nice too.