I don't mind the non-d20 reviews here either. And it's nice to be able to see some of the staff reviewers, since I trust them and already have a feel for their tastes, take a look at other products.
Categorization does sound like a good idea to me. I'd think just two categories, d20 and non-d20, with all books that are d20 logoed, derivative of the d20 system (such as M&M and SAS), or purely generic (like plot hooks books) going into the d20 category. Those kinds of books strike me as things that even someone only interested in d20 stuff will still want to look at. Non-d20 books would only include those books that specifically use some other game system.
One other thing that would be really nice, but I'm not sure how easy it would be to do, is to add keywords to the products listed in the product guide and then have a system for searching by keyword. This would probably work best with a list of pre-designated keywords so that we have some idea what to expect when one pulls up a search for "high level adventures".