A critic's job, to boil it down, is to measure the emotional response a movie has on him or her, and then figure out a way to intellectually analyze this emotional response. Sometimes you'll hear critics explain that it's much easier to review a bad movie or a good movie than a "just OK" movie. "Just OK" movies do little to inspire an emotional response, and the critic must go about his or her duty trying to explain whether or not anyone should go see this movie that everyone will most likely forget.
I've become increasingly irritated by "just OK" movies, and it comes down to this: I would rather see a bad movie with personality than a decent movie with no personality at all. It's far worse walking out of something passionless and mechanical than from something that tried and failed.