Although I don't agree with everything you got some good points. First and foremost I think we need to add a definition of each rating category like you have suggested. Audio and Video quality is not a rating of the music choice but a rating of the quality of the actual sound and visuals. By explaining this better to the users we will hopefully get a less downrate because of music choice.
We have a number of systems in place to keep downraters away.
1. You need at least 2 fame to vote.
2. You can report comments by clicking the red x in the corner of the comment.
3. Reports can generate warnings, if the user gets 3 warnings he cannot rate. The user also looses a lot of fame with each warning.tr
4. We use a weighted system which means a user with few comments and ratings will have less impact on the overall score than a long time user.
We also very recently added the Skill system which is only used for PvP/Arena movies. Check out this movie.
http://www.warcraftmovies.com/movieview.php?id=54037 Even though it has a fairly low rating it still is the #1 skilled movie on the site. This means it has been up on the latest achievements two times as most skilled this week. We are going to make changes so that PvP movies rely heavier on the skill values in the future. For example we will put up at top skilled this week list on the front page with the 5-10 most skilled movies.
As a last note, if you ever see someone downrate a movie because of music; click the red X. Chances are that the guy will get warned or even banned.