James at Grognardia has a post on what genres/subjects that the OSR should focus on as well. I played GURPS and Hero System for a number of years. Those RPGs are universal system with a number of genre books implementing the core system for a specific genre or setting. Based on my experience here is a rough list that should help the discussion along.
Adaptions i.e. licensed setting
The last two, Humor and Adaptions, are often mixed in with the others. For that matter many RPGs are a mix of two or more genres. For example ShadowRun.
Hopefully this list will help people think about what other areas that old school games can cover.