Daniel Mendelsohn argues that the best critics teach their readers "how to think." He says criticism relies on a simple equation, "KNOWLEDGE + TASTE = MEANINGFUL JUDGMENT":
The key word here is meaningful. People who have strong reactions to a work—and most of us do—but don’t possess the wider erudition that can give an opinion heft, are not critics. (This is why a great deal of online reviewing by readers isn’t criticism proper.) Nor are those who have tremendous erudition but lack the taste or temperament that could give their judgment authority in the eyes of other people, people who are not experts. (This is why so many academic scholars are no good at reviewing for mainstream audiences.) Like any other kind of writing, criticism is a genre that one has to have a knack for, and the people who have a knack for it are those whose knowledge intersects interestingly and persuasively with their taste. In the end, the critic is someone who, when his knowledge, operated on by his taste in the presence of some new example of the genre he’s interested in—a new TV series, a movie, an opera or ballet or book—hungers to make sense of that new thing, to analyze it, interpret it, make it mean something.
Robert Butler amends the equation:
That's neat, but it leaves out one of the most important qualities in a critic: their voice. The impact that Kenneth Tynan or Pauline Kael or Clive James made with their criticism is inseparable from the force and personality of their voices. I would refine Mendelsohn’s equation one stage further:
KNOWLEDGE + TASTE + VOICE = PERSUASIVE MEANINGFUL JUDGMENT.
The Dish 