An essay by Anne Burke in the latest Context (published by the Center for Book Culture, which also houses the marvelous Dalkey Archive Press) takes reviewers to task for saying that any book is "not for everyone", because it's a lazy phrase -- after all, what book is for everyone?
M.A. Orthofer and Scott Esposito have both replied, essentially agreeing that the phrase is, indeed, a bit silly.
There's a way that "not for everyone" can make some sense, though -- if the reviewer is thinking of "everyone" not as everyone on Earth, but rather as everyone who reads that sort of review. It's still hardly the best choice of description for a book, but there are bigger crimes in book reviewing. If the reviewer goes on to explain why she or he feels the book is not for whoever it's not for, I don't tend to mind the phrase too much. It's the explanation that is key -- any review where there are claims made about a book that are not supported is a weaker, less useful review than one where the judgments are explained.
Having written some reviews of 500 or 800 words myself, I know how difficult it can be to avoid cliches, hyperbole, shorthand, etc. etc. etc., and other things that are not for everyone. Truly thoughtful, insightful reviews under 1,000 words are rare and deserve to be celebrated.