I just released my fist two commercial WordPress themes: Lucidity and Lucidity Catalog with a focus on simplicity.
After creating over one hundred custom WordPress themes – I decided to take my most requested features, coupled with a slick admin interface, and some really simple XHTML and CSS and put it together to create a really easy to use and customize theme.
If you looking for something really easy and clean – take a look at the CSS used. You can see that a little goes a long way!
Just remember – keep it simple. When you overcomplicate – you will end up with sloppy code, hacks, and problems with old/new browsers alike.