By their very nature, Q&A and forum pages are extremely difficult to optimize for search engines. Most of the content is from your visitors and is difficult to focus on good target keywords. However, discussions are very good for long-tail and mis-spellngs [sic].
We all know the routine for attention grabbing titles, compelling meta descriptions, focused keywords, well structured headings etc, but what truly matters is good content and incoming links.
As a webmaster, you will get much better search engine positioning by keeping discussions on topic than by any amount of tinkering. If you don't believe me, check the top pages for your target keywords. Every time I have done this, half the top pages have none of the expected SEO features that we are all taught are so important (Internet marketers are not going to make much money, if all they tell you is to write structured, well focused content, then go and get some links - they have to sell you a system).
Having said all that, meta keywords (from the tags) and meta description (from an admin only excerpt box) would be nice, but I do not think they should be a priority. (Hint: HTML markup in this box should be :grin:)
For top search engine performance I would say:
* Write a good guide to contributors, to encourage them to stay on topic, and to use good summary questions (e.g. this question is about SEO, yet it is not mentioned in the summary question - what will Google think?). Consider an ebook and video version of the guide, to maximize exposure.
* Add comments as much as you can to enrich the page. Write these for your visitors, but with keyword focused phrases to help search engines know what the page is about.
* Vote up the well structured questions and answers. Vote down the off topic stuff. Hide in extreme cases, but only as a last resort - you can achieve more with good comments.
* Focus your SEO efforts on summary and reference pages, using the Q&A pages for source material, and drive traffic to those pages in your answers and comments. When driving traffic, you should answer the question then add the link, not just write "see article ..." These pages are also much easier to monetize (at least with AdSense) as most visitors are too focused on the discussion to click the ads. Obviously, this needs to be outside Question2Answer - it could even be a separate website - as Q2A, quite rightly, has no page building capability.
* To get started, register yourself as several people (with their own character) and talk to yourself about your keywords all day. Some say that schizophrenia helps here, but I'm in two minds about it.