I use it regularly with a lot of my friends and it's one of my favorite apps. Here's my take: The ease of sending and ephemeral nature significantly lower the barrier for what you'd consider shareable. Imagine you were walking down the street with a friend and saw something that gave you a casual chuckle. Something mildly interesting. If you were in person you might point that out to your friend, promptly forget about it, and carry on with your day. If you are by yourself, it probably isn't interesting enough to save a picture to your phone and send it via text message where the default is the image is immortalized in your photo library and text history until you delete it. Snapchat makes it so quick and easy that those little moments are now almost as easy to share as pointing to your friend if you had been walking together.

I agree that the innovations are not major, but each little nuanced feature in combination makes it so that people share the little moments in life and you suddenly have a small window into the daily lives of your close circle.

EDIT: I should mention that I have no opinions on its viability as a business. Just commenting why I enjoy using it.