Another thing to keep in mind here is that these kinds of review systems will often result in high amounts of polarization regardless of whether the topic is controversial. My feeling is that people often vote based on how they want the current average to change rather than based on how much they liked the movie.

E.G. Sometimes I'll watch a movie, I'll think it's an 8/10 but then see it has 4/10 on imdb, so I'll give it a 10.

Presumably this may affect another person's vote. Maybe someone saw the film, hated it, wondered why the review was so high, and gave it a 0 to bring the average down.

IMDB recognizes this and uses a "secret formula" (http://www.imdb.com/help/show_leaf?ratingsexplanation) as a result.