This should really be called, “How to write a negative review without being a troll.”
The problem with reviewing text is that in addition to finding fabulous writing, one also ends up reading horrible pap. And sometimes it was produced by people you like and respect.
If I am editing a piece, I can help improve the text. When reviewing, though, I can do nothing to improve the text. The only thing I can do is articulate the problems in a way that can help the potential readers.
(On a related note, I sometimes wonder whether there even are readers. I regularly publish in magazines and websites with hundreds of thousands of subscribers, but I very seldom hear from any of them, and most often these are notes from the PR manager at a company I failed to mention.)
Anyway, when reviewing a literary work that doesn’t deserve praise, it’s easy to be cruel, and harder — but more useful — to be thoughtful, balanced, and to attack the weaknesses of the work without lambasting the writer. This is a decent primer, from Salon:
How to write a bad review – Salon.com.