I thought about this problem (finding a good theme) a while back too. I think this one of those nice ideas that doesn't scale, at least not in the current form, or the forms of similar efforts (like the various theme finder sites). In an ideal world there would be a resource to read detailed feedback from in-the-trenches users, because there is only so much information you are going to get from someone who's passing through a theme with a checklist type approach. It's much different than someone who's used a theme for one or more sites and had to actually build something with it. Also, I think perhaps the bigger factor for customers is not just the code quality, but the service behind the theme. A theme that passes all checks and scores well code quality wise is not useful if the theme author closes up show months later (it happens).

I also think the affiliate model is broken for this type of thing. You need a ton of referrals to make noteworthy amounts of income. Lots of shops don't have affiliate programs, so there's a persistent tension between having to review these things even if you try to stay as neutral as possible.