Pistos, welcome, and thanks for the feedback and the ideas!
Re forums: I understand what you're saying. Some things to note though:
One of the nice things about Yedda is that Q&As on a specific topic (say, like Yedda itself) do not "pollute" - you will be exposed to them only if you specify "Yedda" as one of your interests, or if you search for them.
This is happening because Yedda actively matches questions to people, based on your personal interest and preferences.
Having said that, this works well for Q&As - but is still lacking when it comes to brainstorming, commenting, etc - anything that does not fall into the definition of Q&A. So we may need forums after all :)
Re "as-you-type" - coming soon!
Re preview - we added preview when you compose a question, but not when you answer one. You're right of course and it'd be useful there too, we'll add it. However, note that the editor we use is WYSIWYG, with an integrated spell checker, so you don't actually need to preview to spellcheck / see what it'd look like (won't it be hilarious if, having said this, my post will turn out to have 3 spelling mistakes in it? :)
So thanks again, and I hope you'll continue to enjoy Yedda.