We actually have quite a few built-in SEO features: http://en.support.wordpress.com/search-engines/
This includes XML sitemaps, available at [yourblog].wordpress.com/sitemap.xml
CSS errors have no impact on search engine rankings, as bots don't interpret CSS. Regardless, if you do find any, please let us know in our Themes forum: http://en.forums.wordpress.com/forum/themes
We could spend years arguing the merit of meta keywords and other SEO tactics, but the last presentation from Google that I attended suggested that meta keywords were on their way out, and this seems to be the case for the other major search engines as well.
It's a new era for SEO. Content is now the key, as it should have been from the start.