I can tell from my experience that, Wordpress theme is not just about making your blog or site look great. The right placements of advertisements, header and footer information, and meta tagging, alt tagging of the entire site, individual pages, every picture, every animation, video, attachments is of utmost importance. On page SEO is the first thing one should try and accomplish.
Only problem is that all themes including the commercial and free ones distributed are not too well optimized. Bloggers who are beginners tend to get attracted to the graphics of a theme without considering the invisible search engine evaluation of the theme.
Hence between a well optimized theme and just a plain graphical one, the former will win over the latter.
Take wordpress or anyother CMS, on page seo precedes aesthetics.
Even after you are done with it, there are many online meta tag analyzing sites, which will tell you the problems associated with your page, and suggest you changes.
You definitely should look into the home page to begin with because the home page needs to be hundred percent correct.
For example too many links on the home page is also a downer. You may think there are just a handfull of them but on closer introspection you may find there are more than 200 links on home page itself, which is against the basic seo rules. This was just an example.
So on page wordpress theme seo is a must for better search engine rankings. You should read the reviews or check the codes of the best theme you like, before you want to use it for your blog.