Excuse me, Big Brother . . . I find the idea of universal review guidelines incredibly oppressive. Sure, not all reviews can be well-done, but, what can I say, it's the Internet. Buyer beware, as always. Sure, we'll inevitably get badly-done reviews (there is one review site where I find the reviews consistently lacking in information about the product). In my opinion, reviews are mostly for getting information about the product so you can decide whether you want to buy it or not. My reviews are probably not as well-known as, say, Alan D. Kohler's, but I do generally get hundreds of hits on most d20 reviews (note: please, no one take an average of the number of hits of each of my d20 reviews and tell me I'm wrong; I'm just guessing). And at AtFantasy we don't even have a numeric system for reviewing (it is a bit misleading anyway)—I just say what I like about the product, and what I don't. I try to keep it short because I hate reviews that are too long (if I read them at all, it's at work). But mostly the opinion part takes a backseat—true, it's there, but the main purpose of reviews (why I read them, anyway) is to get what's really in the book (which is often not provided by publishers). For example, a publisher may say "30 prestige classes" (or whatever) in a product, but it's the reviewer's job to say what they are, or at least some of them. When there's a great number, however, that becomes prohibitive, so a few examples should suffice.
But as far as review standards go, that's probably just going to suck the life out of reviews, particularly coming from a publisher (Nat. 20 Press).