Time travel movies can be really tough to do. Getting a sense of how actions in the past can affect people in the future is hard to show effectively, and usually it's pretty ham-handed.
This movie did it masterfully. And it did it with a style that was stark, realistic, and poignant all at once. The vision of the even the near future that we see in Looper is not a great one. It's desolate, but definitely a future that makes sense to us - people living on the streets, dirty cities, familiar technology with just a sprinkle of futurism (the flying crop-spraying robot, for example) all made for a great setting.
The plot itself was interesting and understandable, but just far out enough that it was still sci-fi. The acting was outstanding - Levitt was great as a resigned-to-his-fate hit man, and Bruce Willis was amazing as his driven future self come back to fix problems that plagued him in his life. The dialogue was simple but well-directed, and everything flowed nicely from scene to scene.