While Brick had tons of The Maltese Falcon and Cowboy Bebop references and undertones, Looper is great because although it has a noir feel, at the same time it doesn't feel nearly as derivative of anything.
Looper doesn't waste too much time with unnecessary explanation (a brief promise of the foray into time travel explanation is interrupted by an impatient Bruce Willis -- who plays the future Joseph Gordon-Levitt -- who doesn't have time for it), and this movie is stronger because of it. Johnson's best stylized shots are the ones that don't show everything -- a fight we see from the outside of the walls; a gruesome surgery that we only see one side of; the aftermath of a massacre. He knows exactly what to show and what can/should be left to our imagination...Much like his plot and world-building.
Johnson's script and dialogue are smart and funny at times. It's clever without letting itself be overcome by its own creativity. This is good writing. The voiceover narration can be a bit jarring and overdone at times, and it's odd how it completely disappears in the middle of the movie when it's so prevalent in the bookends of it, but that's a minor critique.
The only issue I had with Gordon-Levitt's character was that he was so well-established as a kind of jerk, it made his altruistic actions much less believable. The movie unsparingly raises some moral questions as well, without overdramatizing them -- when do the ends justify the means? Is it ever legitimate to kill for the ones you love? How about to kill to prevent hurt to the ones you love?
Overall, Looper is a well-wrought world. The more I think about it, the more plot holes I come up with...but also the more I'm impressed with how Johnson wove a rich tapestry. This isn't Matrix, and I hope you're not expecting/hoping for that. You have to appreciate Looper for what it is and what it aimed to do. It's good cinema in a Hollywood-drenched present. I really hope to see more of Rian Johnson in the future.