If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.
What makes brainstorming specifically different from, say, any given discussion in a comment thread or on an Open Thread?
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.