Right now, IPB supports a single heirarchical classification of Topics:

Forum (category) --> Sub-Forum (category) --> ... ... -->Forum --> Topic

One feature I would find useful is an alternate Category/Classification Tag applied to Topics.

Here's an example: in a forum where the heirarchy is organized mainly by software-category, I would find it helpful to be able to tag some topics as "Rant", "Newbie Question", etc. One could extend the idea to allow multiple tags for a topic, but for my purposes, a single tag would be just fine.

The use of the tag would be in Search, List functionality. Most importantly, I'd like to allow individual users to opt out of topics that have certain tags, or to opt in only to specific tags.

It sounds like something best left up to a modification.

Or perhaps if you're moderators are all global moderators, you could make members "moderators" on all forums with permissison only to multi-moderate and have a multi-moderation that tags topics.
Not ideal solution but a feasible one.