Design for me. I believe this is the most important factor when initially selecting and purchasing a theme. I doubt most theme buyers are going to actually look at the code quality (unless they're technically savvy). They'll just buy it, and if there's a problem, ask for support later.

I generally don't need support when it comes to WordPress themes, but I can imagine that would be another major factor for people just starting out.

Price isn't a huge factor for me as long as it's reasonable. Obviously I wouldn't want to be spending a ridiculous amount of money for a theme lots of people are going to have anyway.

Personally I don't really care about the license either. Sure GPL themes are the least restrictive, but if there's a really good design that I want to use, I'll live with it.

Business reputation is an interesting choice. I can certainly see myself being more likely to buy a theme from one of the big names rather than a site I've never heard of before.