I came across this link to an article by Bill Ward on the how and why of reviewing on Grasping for the Wind. Like John, I like it a lot. I had to write an awful lot of review to figure even half of this out for myself (and I continue to ignore this hard won knowledge on occasion).
It's also nice to see this subject handled in such a constructive way. I have seen a lot of essays of reviewing that basically say most reviews suck and go on detailing why. Usually this comes down to reviews not containing what the essayist feels it should contain and the whole thing spirals into a pointless debate on what a review actually is.
Ward does not go for a precise definition in his article but he does mention why it is so bloody hard to write a proper review. You are trying to please more than one crowd with one text. I think it will take a couple of hundred reviews more on my part to really get the trick down. Ward offers some advice to reviewers and aspiring reviewers. Recommended reading.