I'll just list the dumb:
1. It is 2044, the future. Young and old versions of Looper character look nothing alike - despite some extensive make-up. Joseph Gordon Levitt has a wide, triangular face and Bruce Willis the perfect oval egg shape.
2. Looper character is basically a douche-bag with no redeeming qualities, so don't know why an audience would like him
3. Young and old versions occupy the same space and time and even touch but don't zap each other into oblivion amid a shower of sparks for some reason.
4 . Production design and set-dressing are terrible - cheap-looking and unimaginative attempt at a dystopian future ... really terrible, especially the street scenes in the city.
5. Obvious, cliched or gimmicky atmospherics: driving a Miata, wearing ties, eye-drop drugs, 90s nightclub, retro roadside diner, and the blunderbuss.
6. Farm Woman lives all alone with the Most Important Kid in the World (MIKW) and must shoo away trespassers regularly but leaves the house and barn open and lives her life like she's in a pharmaceutical commercial.
7. Older Looper is determined to make sure he can re-set time so his lady-love won't get whacked 30 years hence in his future-future-past 2074, yet doesn't see as his highest priority protecting his younger self when it has already been established that the mob bosses from the future want first-and-foremost to kill the younger version because that erases the older version. He could have just chained Young Looper to a wall somewhere and done his business and come back to free him later.
8. Time-Machine looks like a leftover piece of junk (a pressure-chamber with hoses and wires) in an abandoned industrial warehouse - and why wouldn't the future mob bosses send their victims straight to the bottom of the sea or into the incinerator and skip the Looper middlemen (and paying them)? Yet another of Gen-X, Gen-Y, Millennial aesthetic choices in film to make a dull-looking future and special effects (K-PAX, Children of Men) with an insulting simple blur for the time-travel effect.
9. MIKW has telekinetic powers - woo. Turns out other people do to, but this is just a plot device to set up the kid's character who will somehow change everything in the future future future ... so the story veers from older Looper wanting to re-set his past to make sure he can live happily past his 30-year expiration date to needing to kill MIKW because he'll destroy everything in the future future future. Dumb.
10. You've seen it all before, in Outer Limits episodes, Twilight Zone episodes, and various sci-fi films.
Random Dumbness: Kid shows younger Looper secret tunnel to hide from hit-men at farmhouse but they expose it and exit from it once the immediate threat is gone; Farm Woman is so worried about assassins and regular homeless/vagrant intrusion onto her property that she is always swinging an axe at a tree-stump and wielding her rifle and being alert, but has time to sit in her house with the doors unlocked and open and beckon young Looper for some sex in the middle of the night; a Miata; an assassin falls in love with a hardened hooker who seems to have zero feelings?; everyone misses at close range when they fire guns even after one character talks about how accurate his aim is with his favorite weapon; jet air-bike thing; young Looper goes back to his apartment right away once he finds out his mafia boss wants him dead; Farm Woman takes forever getting truck loaded to escape and drives off with gas nozzle still hanging out of the truck (full snark on that one: a key example of terrible prod. design choices); KIKW flips turck and nearly kills his mom and himself rather than just telekinetically making older murderous Looper's weapon go bye-bye.
#11, and the pièce de résistance: Young Looper decides suddenly to be noble and break the loop by killing himself when nothing in his character nor backstory, especially the future-past life of his older self, would lead you to believe he had that in him or would ever do it. Further, the kid already TK'd a blast wave and why wouldn't he just wish older Looper into the cornfield as he kept coming anyway?