This post from Coding Horror talks about a subject near and dear to my heart: the usability of the discussion boards. Here is my take on what constitutes an easy to use environment:
- Flat discussion works best if few replies are expected for a given topic. You can make sure of this by splitting up the conversation by topic.
- For returning members it is very important to quickly find out what's new. While the method of color-highlighting commonly used kinda works, the best way IMHO is to create an RSS feed for your forum.