Forgive me if this has been covered already, as it seems likely it has...
why not include category information in the class for items in the discussion list? for instance, right now all discussions are broken into DL elements on the main list, and each of those DLs is given class info like <dl class="Discussion Read NoNewComments NoReplies"> A bit later on you have the link to the category that discussion is in, like <a href="./?CategoryID=6">Plushy Porn</a> but as near as I can tell that category ID isn't reference anywhere else that might be useful.
Well, if you changed that and added the category to the DL class, like so: <dl class="Discussion Read NoNewComments NoReplies Category6"> (... or something similar) then it would be possible to do some nice little display tricks, like including a different icon or subtly different background/border color for each individual category. That way, looking at the master list, it would be instantly possible to visually filter information without having to click anything.
Please now tell me the various problems with this idea that I have overlooked. ;)
The only "problem" I can think of is that semantically it would over-rule the existing meanings. But being that it would be your forum, you could decide to ignore the standard "new" or "not new" meanings to the colors on the discussion index.
As an extension, this is a bit of a pain to do. I would have to add that to the central codebase - which I'm not opposed to doing.
Well, my thinking is that the current system of using color grading to indicate read/unread/new and so on is rather nice, but what I would probably do with this ability is add something like a 4px left border to each discussion in the list which would be colored appropriately.
I also reckon a cross-breed of discussions & categories would work... well for forums with a small number of categories anyhow.... with maybe 10 recent posts in columns, under each category heading...