I think the main thing is the confidence level it takes to post something. In most conversations, people only seem to post when they're relatively confident. In brainstorming, you could propose ideas that you're not as confident about.

Another thing that is sort of different is the intention. In brainstorming, the intention is to explore different possibilities in search of finding a good solution. In normal discussions, it seems to be mostly about sharing information and discussing the logical inferences people have made.