Works for me - Sometimes I feel like I've been blind-sided when I'm not expecting a horribly depressing situation, a'la (SPOILER ALERT) "Million Dollar Baby" and "House of Sand and Fog". Both wonderfully well-acted but extremely depressing. Not sure how to label all movies without giving too much away in some cases, but I totally understand what you mean. I hope others will "chime in" on this topic.
I like the idea, but I think there should be more categories. I.e. Black comedy and melodramas, which might not make everyone feel the same way. But I think "feel-good" and "sad" categories are a good way to start.
The categories would be separate from genres, but more prominent than a bunch of keywords most people will never look up. I think this feature would help find movies to watch, and make the site more social as it would surely rely on user input to categorize titles.