- Adding ALT tag to all images. Markdown does not support alt= at all.
It does, actually:
![alt text](image link)
- Intelligent construction of keywords meta tag
It doesn't really seem like Google and Yahoo have cared about the meta=keywords element for a while. Presumably Bing follows the same pattern, although if your forum is publishing news content you could try using the new news_keywords meta attribute.
- Fine gained control of how Title element is constructed
- Google has recently announced they would also use rel="next" and rel="prev" link elements in the head section of your paginated archives.
There's your plugin! The latter of those could just be left to the theme though, but the iirc former would definitely need to be plugin-ized since themes don't allow for easy configuration from the front-end (yet?).