Steven, there are many paths to coming up with a strong story concept. One approach: think of a story idea at the most granular level - the conceit. What is that *single* story element which lies at the foundation of the concept? For example: Groundhog Day --> Living the same day over and over. Inception --> Implanting a thought into a character's mind. Not all movies or TV series have this kind of hook, but it sure makes it easier to get a buyer's attention.

Re Chasing Amy, at the time, the buzz was about Amy being a lesbian who gets involved romantically with a guy (Ben Affleck). In 1997, that was pretty provocative.