Bryan Caplan's The Myth of the Rational Voter is a short, brilliant book. He described four ways that the average voter is biased as compared to the "enlightened public," a group who has more factual information about how the government works.
Because I like his ideas so much, I've copied them and applied them to movie rating. Here are the four most prominent biases that affect your typical movie rater:
1) Availability bias: Nobody on earth has seen every movie ever made--there simply are not enough hours in the day, given our current lifespan. Individually, each of our relative rankings must be flawed because we inherently have imperfect information.
2) Snobbery bias: I am always surprised at the number of people that will automatically disdain a movie because it's too low class. People generally only watch the best movies--you'd be surprised at the percentage of movies in the top 250 that are in the average DVD collection. So when a movie is in some way not "great," it is disdained. The effect of this is that many raters lump movies as either "incredible" or "terrible," without the necessary continuum.
3) Anchoring-based bias: If a movie wins an Oscar, the average rater will assume that it must be at least pretty good. If a movie is already rated highly on imdb, subsequent raters will give it higher rankings as well. These are both sad but unavoidable consequences of the rating incompleteness theorem.
4) Anti-foreign language bias: Dubbed films are awful and subtitles annoy many people, so this bias is a natural consequence of the unfortunate fact that the world is not yet filled with anglophones. Memo to the rest of the world: hurry up with those English classes!
Availability bias and snobbery bias are mitigated in large part on imdb because of the diverse voting population. Snobbery bias hurts a lot of professional movie critics, in my unprofessional opinion. Snobbery bias also explains why you see so many 10s and 1s on imdb. Anti-foreign language bias is a cross-cultural phenomenon, and it probably hurts imdb overall because so many speakers are from the United States. But there are some foreign language films on the Top 250, and the site offers over 500 languages, so there is hope yet.
I was tempted to include an "anti-black and white bias," but I decided that this is actually overplayed. Look at the success of Sin City on imdb, especially with the young raters that are supposed to be so resistant. Those under 18 gave the movie a 8.7, although I will admit that I have not accounted for the Jessica Alba factor.
What biases am I missing?