Ideas:
1. Suggest movies from your "haven't seen it" picks that you might like based on your votes. You actually have a big advantage over Netflix and Amazon here because you have users telling you what they haven't seen. All the big companies know is whether the user has or hasn't bought or rented the movie.
2. Make themed subsets to vote on. Do a great 80s movie experiment. Or a great sci-fi movie experiment. Put up a new themed set each month or quarter. At least there will be something new on a regular basis then.
3. Let users see a personal top 10/100/1000 list based on their own votes. Of course, they will need to vote a LOT for this to make sense, but that's kind of what you want, isn't it? Maybe give them a percent of votes needed to get good results, like, "You need to vote about x times to get good results. You're at 35%, keep voting!"
4. This is vague and useless because I don't know enough about Facebook to know what would work, but make something that users can share via Facebook. Think how online quizzes get more traffic by giving badges that users can post on their wall.
I hadn't realised until I read your comment that the "logged in" features of the site aren't talked up on the front page. When logged in you can see your personal top films based on your votes, and your reccomendations based on films you haven't seen. These are currently only recommended based on their rank, but in future we'd like them to be weighted based on your voting history and a method of matching you to users who voted in a similar way.