Interesting working paper on student evaluations of professors. While the author is particularly concerned about evaluations in law schools, a lot of the research she cites seems to apply to evaluations in general. Consider this:

Conventional student evaluations strongly mirror the professor’s smiles, gestures, and other mannerisms, rather than the professor’s knowledge, clarity, organization, or other qualities more clearly associated with good teaching. The way in which a professor walks into the room or smiles at the class can affect student ratings much more substantially than what the professor says or writes on the blackboard. [footnote omitted] Evaluations collected from students after no more than five minutes’ exposure to a professor accurately predict assessments gathered at semester’s end, leaving little doubt that these evaluations reflect relatively superficial behaviors. [footnote omitted]

To paraphrase Billy Crystal, "It is better to look good than to be good. And you look maahvelous!"