An SEO plugin is sorely needed and with the little coding I can do, I've started developing one also. Unfortunately, I'm not a good coder, so it's not something I would release out in the open. I think all your suggestions are good ones and I've already hacked together some of them. Some I might add are:
1. Have one category page per category, instead of a different category pages with the same exact content, depending on where you are on the Q2A site. This was brought up on another post on this site (http://www.question2answer.org/qa/7632/duplicate-content-issues).
Well crafted category pages that are unique are very helpful for SEO, but the way Q2A puts together category pages now is, IMHO, silly and serves no useful purpose, especially if you want to create solid landing pages for SEO.
2. Allow for meta descriptions for tags pages, and also allow for this static description of the tags to be on the page on top. The tags pages can do very well for SEO, too, but the inability to control some static content on the tag pages is a minus.
3. Whenever you edit a question title, Q2A immediately changes the URL (if you are using SEO friendly URL's). This is not a good idea, since if you are involved in SEO you may update content frequently - but then the URL may change all the time for the post, if you decide to change the title of the post after doing some keyword research. This is not good. The URL should be saved once upon post creation, and if you want to change the URL slug afterwards, you should be able to do this, but not by just editing a post. See how this is done in Wordpress. I think Q2A needs a URL alias table maybe?
4. Also, something I've brought up before also, do not allow users (except for admins) to edit, hide, or delete questions. There is no useful purpose for Q2A's built in function that allows users to edit their own questions for as long as they want or to hide them whenever they want. Either remove this feature entirely or set up a time delay for this.
5. FYI, meta descriptions are not useless, but your suggestion to specify your own meta description for questions is the right way to do it (I've set this up on my own by using additional custom fields).
I'll add some more as I think of them.