Amazon has a dedicated page called Prime Member Exclusives: Not on Netflix that lists every movie and TV show you can stream via Amazon Prime Instant Video, but not on Netflix. So, if you have both services and are looking to find something new to watch that's not on Netflix, there you go.
The page is not exactly advertised and a similar function isn't available on the Amazon Prime Instant Video app, but it's there. Apparently, there are plenty of people who don't know that this list exists. A post on reddit suggesting that "Amazon Prime Movies should have a category called 'Not on Netflix' for people who have both services" has quickly climbed to the front page in the past few hours. Of course, the top comment is someone linking to the exclusives page and saying It does.
For people who have both services, it's a pretty useful tool. As of right now, there are 307 titles to peruse.
Most of the top picks are Amazon originals and HBO shows, as HBO content is one of the big advantages Amazon Prime Instant Video has over Netflix.
So, there you go. A quick, easy way to see what Amazon Prime can offer you that Netflix can't. Carry on.