You hit rock buttom to one of the most asked for features. Reply to forum postings. Forums should be able to replace simple listserv mailing-lists. And it should actually work and be documented how to get it to work.
Regarding use cases, it is necessary to be *aware* of multiple uses for the design of the software, but they do not have to be fully fleshed out.
Last but not least. Do not reinvent the wheel over and over again. Jeff posted links to the state of the art. Mimic it. This should take care of the usability that concerns most of the users.
If you want to get serious about Forums, then take a look at the forum developed for the StudIP plattform.
http://www.goettingen.studip.de/forum.php?&flatviewstartposting=0#anker
The forum package (on it's own) has been the topic of a PhD thesis from a sociologist and has a ton of features that make the forum useful even in a very complex discussion. I'd change some of the things, but if you want to extend forums beyond a basic approach, seriously take a look at it.
Summary of key extensions (which I found really neat):
- Collapsable Rating and Poster information
- In posting quotes (the quote appears in a seperate box within the posting).
- Relevance calculation (based on views, rating, links)
- Indicator in the front of each posting (esp. in collapsed view), for immediate color coded scanning of relevance, rating or new postings.