Since you have stated you do not want to introduce categories how about layers of tags  - a primary tag and a child tag to help organize the posts. The primary tag would help group the discussions and the child tag would organize the threads under that primary tag. There are some forums that have such broad discussions the listing of all discussions and tags is overwhelming to look at. For example:

Science Activities, Class405, HomeworkClass405, News, New Member Form, Arts and Language, Resources, New Member List, Kids Activities, Teen Get togethers, Meetups, French Homework, Math Activities etc If there are a lot of different type of discussions this would result in ALOT of tags to search through if you don't know specifically what to search on. And if you go to the search tags you will be scrolling through a lot of tags. But if you could have a Primary Tag (the admin could restrict these).

Classes, Activities, Meetups, New Member Info, etc.

Then lets say under Classes then the child tags would show under related to the Master/Primary tag: e.g Classes - French Class Roster, French Homework, Arts and Language Class Roster, Science Class Homework). This would be a much easier search for the end user.

The Master/Primary Tag would help visually (some people are very visual). 

There could be an option to have a Master/Child tag under the tagging section. This would truly make Live Forums flexible.

My background is software user experience- people like to organize data so it isn't overwhelming to find something. Our brains are setup this way - information is compartmentalized. Having free flowing tags can be very overwhelming to the every day user.