I'm the Executive Editor at Mathematical Reviews. I'm impressed by some of the answers, particularly those from Timothy Chow and Denis Serre, which give as good an answer as I could hope for. As Timothy Chow and Joe Silverman point out, having links to other items in MathSciNet related to the current paper is especially helpful to users. Kimball quotes the salient bits of our Guide for Reviewers. Thank you for doing that. People tend to forget that the Guide exists. When I talk with people in person at a conference, I compress the goal of a good review down to: describe something about the context of the result (where did it come from?); describe the main result (what is it?); describe the main technique(s) or method(s) (how did they do it?). Doing all this concisely can be difficult, of course.
The AMS Bulletin still publishes reprints of reviews. They are generally tied to one of the articles in the issue. While they are generally above average, they are not necessarily meant to be the best of the best. For a time, I was posting particularly good reviews that I came across. You can find them via this link: exceptional reviews. I should probably get back to posting examples of good reviews. I second Yemon Choi's recommendation for Kimball's collection of (his version of) exceptional reviews.