Here's an excerpt from a Richard Schickel piece for the LA Times:
Criticism — and its humble cousin, reviewing — is not a democratic activity. It is, or should be, an elite enterprise, ideally undertaken by individuals who bring something to the party beyond their hasty, instinctive opinions of a book (or any other cultural object). It is work that requires disciplined taste, historical and theoretical knowledge and a fairly deep sense of the author's (or filmmaker's or painter's) entire body of work, among other qualities.
Opinion — thumbs up, thumbs down — is the least important aspect of reviewing. Very often, in the best reviews, opinion is conveyed without a judgmental word being spoken, because the review's highest business is to initiate intelligent dialogue about the work in question, beginning a discussion that, in some cases, will persist down the years, even down the centuries.
You can find the complete piece here. (I had some trouble setting up a link before.)
I'll add some comments later.