Even before instagram allowed full size photos, I was using another app to allow the whole photo to be edited, and then cropping the photo after, and then printing them out. My entire apartment is decorated with framed photos that I edited slightly in instagram, but you'd never know because the edits are very subtle. (to be clear i never even posted these photos to instagram, just used the app as the editor)
If you've ever edited a photo in instagram you realize the difference between the end result and what you started with.
Giving what used to be only reserved for people with expensive software and fancy computers to regular people (and making it easy), is definitely a huge piece of it too.
IG is good because of that and this pushes IG to be more about experiences over BS news and meme-macros that set a FB feed into garbage. (Depending on the person obviously.) But the incentives for interaction are very different than those with a somewhat more "traditional" social app like FB. Furthermore it lacks the update nonsense of FB such that it doesn't feel like a chore to keep up with updates. The stream passes and what you miss you miss.
You only upload 1 photo.
True, someone can dump a lot of photos one at a time, but I think that's a bit of an instagram faux pas. It tends to be that if someone is on vacation (for instance) they don't dump all their photos on instagram, they choose their one best photo, crop it, add a filter to it, and try to make it great. You don't create an album like you do on facebook. This makes the quality of what you see in your feed a lot higher than what you would see on facebook. I've been told by a few heavy users of instagram that this is what they love about it.