There is zero content discoveries features, so you have no way to get new followers or find new people to follow. And the top posters are all teenage pop stars that I've never even heard about who post completely uninteresting photos.
The more Instagram turns into Facebook, the more this opens up a spot for another company to build an actual social network around photos. Whether this will be 500px, Flickr (again), or somebody else entirely, I don't know.