I love the concept of tags in movielens. But it only works if enough people use them. I think one way to increase participation would be to place the tags at a more prominent place on the screen. They should be visible as soon as you open the movie page. At the moment you have to scroll down to see the tags. Personally, it took me a while to notice the feature and I think some casual users might completely overlook it.
Perhaps you could place a tag cloud next to the rating and movie info, along with an invitation to add your own tags.