Deus ex machina.
I like when a movie sets up a set of rules that it's going to play by, and this is especially important in time travel movies. It doesn't have to be logically consistent, but if it's not, you have keep things moving so that there's no time to dwell on it during the movie. If it is logically consistent (and keeps moving!), then you have the makings of a classic. E.g. Primer. Something that rewards repeated viewings.
Looper does a pretty good job of setting up the rules of its time travel universe, and playing by the rules. Building an alternate universe, and a culture that goes with it. But then it throws in the deus ex machina and tosses out the good will it built up by playing by the time travel rules. That's playing dirty.
I'm still thinking about it though, so that's a positive. But mostly I'm thinking about what went wrong, and how it could have been a good movie. Can't recommend it, except maybe as a Netflix. Watch Brick instead. Or Limitless, which I thought was pretty good. Logic holes, or at least character development holes (Would a smart guy do things with consequences that predictable? But he took the drug, so he's not always smart. Etc.), but moved fast enough to get me past them. I had to watch it again to check them out.