Bruce Willis and Joseph Gordon-Levitt play their parts well. JGL even goes so far as to copy Willis' mannerisms. Emily Blunt hides her accent well enough, but-as with any foreign accent-can't quite hide it entirely. This isn't a deal breaker; she performs as good as she has in other films. Surprisingly, the child is a very competent actor for his age.
The world that has been created is one of subtlety. The director and writers present a dying, miserable world without over explaining it. There is some needless exposition, but not enough to ruin the overall feel.
Time travel is a moderate part of the plot. It works well within the film's established world and isn't over explained. This shouldn't be a problem for most people, but if you start comparing their time travel to others, or try to work out the minutiae, you'll find some holes. No time travel plot line is perfect.
Looper is well worth the hype and critical adoration it has received. If action films with a great plot are your thing, then I highly recommend it.