Do user ratings of products — games, films, books, restaurants, etc. — tend to follow any particular distributions? If so, what form do they commonly take? Are there public data sources that show these distributions (e.g. for Amazon books, Netflix movies, etc.)?
Here's a post from the YouTube blog that, anecdotally, I think is fairly representative of star ratings systems online generally.
As you can see, it's a J-curve.
Here are some further examples from different sectors, with the graphs reproduced here:
The biggest relevant dataset I'm aware of was the one Netflix released for the Netflix Prize competition – however it has since been removed due to a privacy lawsuit. You may however be able to find copies of it online.