It’s really hard to answer this because there are so may different things to consider but the things Black_Coffee_IO, webfector mentioned coves the basics.
Things like schema.org markup for content, page load times are very important. Some of the stuff mentioned in these beginners guides should help you get started.
As far as meta description goes it does NOT have a direct impact on SEO , but having proper meta description can have a positive impact on CTR ( click through rates ).
Google does NOT use meta keywords for ranking , they have not used it for years. FYI having the keywords in the content as sub headings and within the body of the article does help, but that has more to do with the content and not how the theme is structured.
I have an article on the Market Blog about this : http://marketblog.envato.com/general/seo-best-practices-for-authors-on-the-envato-marketplaces/
Most of the time popular SEO plugins covers a lot of these things, so make sure your theme plays nice with them.
Hope this helps