This is a post I made a while back in my personal journal. I figure it's worth reposting here (with a few edits, natch).
We get about 160–180 words per review (it used to be more, but the magazine was redesigned with more white space, which meant reductions in our actual content) and every one of them needs to serve a purpose. These are three extremely general concepts designed to make reviews both shorter and more information-dense.
Death to "to be"
"The passive voice is boring. It should be avoided." -> "Avoid the boring passive voice."
I won't go so far as to say that I don't permit any forms of "to be" in the reviews I edit, but I come down pretty hard on it when I see it. Not only does it weaken the review, but it takes up a ridiculous amount of space. I would say that perhaps 40% of my editing is moving adjectives around, and this is the most common situation that requires it.
"Campbell is tall and strong and likes to sing." -> "Tall, strong Campbell likes to sing."
This is a particular construction that I see all the time. In a publication that omits the serial comma because all those commas add up to a lot of column inches and ink, two consecutive "and"s are inexcusable. They're also unnecessary. (Replacing the first "and" with a comma is also incorrect, of course, as "likes to sing" is not an adjective.)
"This is an elegant fable that has lots of well-drawn characters in it. The setting descriptions are sketchy but still intriguing." -> "This elegant fable, full of well-drawn characters, lacks strong descriptions of its intriguing setting." -> "This elegant, character-driven fable lacks atmospheric detail."
"This is a noun that verbs. It also verbs" is a big flag that two sentences can probably be combined into one that begins "This noun, which verbs, also verbs". Even better is "This adjective noun verbs". Don't get attached to your words; ruthlessly replace them as necessary. "Atmospheric detail" doesn't mean exactly the same thing as "setting descriptions", but it gets the idea across.
The critic's job is to criticize
"This book from an award-winning author follows the adventures of two plucky teenagers..." -> "This uneven third Plucky Teen escapade (after 2006's The Plucky Teen Adventure) from Nebula-winner Jones follows series heroes Mike and Micaela..."
Sometimes our reviewers get caught up in plot summaries and forget that their job is to say something that can't be found in the jacket copy. Unless the review opens with some sort of pithy play on words, there must be a critical description somewhere in the first sentence and another one in the last sentence (which theoretically summarizes the rest of the review, though in practice that doesn't happen much). One of my favorite parts of my job is coming up with precisely the right adjectives. Every connotation is considered; for example, we don't call a book "stellar" unless we're making it literal by giving it a starred review.
Note that it's the uneven third escapade, not the third uneven escapade. It's fine to compare to the author's past work, but only directly review the book you're reviewing.
Who are you and why should I care?
"Ivana and Hoos fall in love, but then Hoos, Egbert, Marv, Cindy, Luis and Hans go to Boringland, battle sentient mushrooms, get sunburnt, argue, trim their nails, and eventually stumble upon the Plot Coupon." -> "Just as hard-bitten soldiers Ivana Bealone and Hustani 'Hoos' Yermamma confess their love, snotty Prince Egbert commands Hoos to join his quest through the swamps of Boringland for the long-lost Plot Coupon needed to cure the Prince's ailing German shepherd, Hans."
Names and places are useless without context. It's very tempting to cut that context in the interests of space, or on the assumption that the reader has read earlier books in the series. Do not give in to temptation! Instead, cut long lists of minor characters, locations, and plot points. Give full names and professions the first time you mention characters, and be generous with the adjectives. Don't worry about hitting every single plot point, and definitely don't spoil the ending; it's good to leave some surprises.
EDIT: Greetings to all my new readers over from LiveJournal! You can read my blog posts via your friends page if you like what you see here. Feel free to look up my personal LiveJournal too, if you like; I'm there (and everywhere) as 'rosefox'.