These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crosses the Line Twice: Jennifer is bound with ropes in preparation for a human sacrifice. The lead singer of Low Shoulder, having invoked a demon, raises the knife... and the whole band breaks out singing "867-5309/Jenny" over her screams.
Hollywood Homely: Amanda Seyfried as the "plain jane" Needy. However, the narration outright states that when they go out, Needy dresses herself down so as to not steal any of the spotlight from Jennifer, and is specifically forbidden from showing any cleavage, since Needy's is more substantial than Jennifer's. This means that, in-universe, the characters are entirely (if tacitly) aware of Needy's actual hotness.
Also Jennifer. If she doesn't feed on schedule, she starts getting ugly... meaning "still Megan Fox but with heaps of makeup to give her shadows and sallow skin." Very likely intentional, as Needy describes this as being "ugly for her", to show how high Jennifer's standards are.
Hype Backlash: A substantial one had already built up against the film months before it was even released, mostly from people who thought Diablo Cody's work on Juno was massively overrated. It didn't help that at around the time of the movie's release, Megan Fox made some rather ill-advised comments about Michael Bay in an interview, further intensifying the backlash from those who thought Fox herself was overrated in various different ways.
Jerkass Woobie: Jennifer. It is possible there is more then her letting it on, allowing her general jerkassness run her life. Also her getting brutal murdered is another factor.
Les Yay / Foe Yay: Jennifer and Needy's friendship is absolutely dripping in it. The bedroom scene takes it way past subtext. If you were under the impression that the trailer exaggerated it, it didn't, it actually toned it down.
Come to think of it, the girl who told Needy that she's "totally lesbigay" in the opening had her eyes practically glued to Needy just before dropping the bomb on her.
Memetic Badass: Chip. He survived getting eaten long enough to get to dry land, impale Jennifer, and even calm his girlfriend down over his own death. If he were the star, the movie'd be over earlier.
Except, of course, that he acts blatantly stupid beforehand - Arbitrary Skepticism, walking on his own through the dark when there's a serial killer on the loose, instantly believing Jennifer's story about Needy and letting Jennifer lead him to an abandoned swimming pool, A.K.A. the ideal murder location. Needy may not be the brightest character either, but at least she isn't Too Dumb to Live.
Protection from Editors: Depending on who you listen to, Diablo Cody either had this directly from managing to parlay her success with Juno into getting a "no-rewrites" clause for this film, or indirectly as a result of the 2008 WGA strike meaning there wasn't enough time for another writer to come in and fine-tune the screenplay.