It would be interesting to see a thumbs-up/thumbs-down rating combined with an optional tag classification system.
I don't think the average user will ever really tag anything. But as long as there's a sufficient quantity of interested users who would, I think you could get a lot closer to statistically guessing why people might like/dislike certain videos.
I like this idea a lot. I also think that once you have some tags, you could extrapolate which movies are likely to fall into those tag categories by seeing which movies people browsed on the site at the same time. You can get this information if you offer one additional step beyond flipping through thumbnails - such as watch a preview. Then with each browsing session you collect data you can use to map genres.
The raw data simply doesn't show a decent distribution of ratings. People are far more likely to simply give something 1 star or 5 stars.
I would say that it's far more meaningful to use a simple thumbs up/thumbs down system...