Ambiguous Disorder: Jessica. Seriously she displays a good few sociopathic tendencies given that it's her idea to cover up the murder, she kills Garrett without a second thought and attacks Maggie for supposedly sleeping with her boyfriend. Add that her snarking at every situation involving the killer.
Asshole Victim: Almost everyone really; maybe one or two of the victims are somewhat sympathetic but most are clearly horrible people. The killer even explicitly details this as part of his motive. He says this is especially true in Mickey's case. The others had to die because they knew the truth about Megan's death, he admits that Mickey might not have known, but was too much of an asshole to live anyway.
The plot is set in motion because the girls wanted revenge on Garrett... except as one of the sisters points out to Megan "Didn't you cheat on him first?" The-soon-to-be-murder victim contemptuously waves away these "details".
Bitch Alert: Each of the sorority sisters from their opening scene. Of course Cass, Ellie and eventually Claire turn out to not be as bad.
Bloody Hilarious: Cass and Jessica find Megan's decayed body in the shower. Jessica's response? "She doesn't look too good".
Even Evil Has Standards: Cassidy's boyfriend Andy is revealed to be the killer, and that he killed everyone so Cassidy would not be charged with Megan's murder. When he asks if she is with him, she says yes. Until he says their going to kill Ellie, then she immediately objects. Though to be fair Ellie was clearly nicer than any of the victims, and Cassidy was probably saying that to placate him anyway.
Expy: The killer's appearance and motivation brings to mind Ben Willis.
Idiot Ball: The entire group of girls. First because they fake a girls death which leads directly to her actual death and second because they decide to blame the murder on Cassidy because she wanted to confess. This goes hand in hand with the Idiot Plot.
To be fair, one of the girls doesn't want to blame the actual murderer because he's her brother and it was an accident, which could be sort of justified. However, every other character immediately agrees with her idea for no explained reason.
Girl #3: Cover up your brother's murder of our best friend just because you tell us to? Sure! I don't see why not.
Not really, it's closer to Jessica, Claire and Ellie wanting to pin all the blame on Garrett but Cassidy and Chugs standing against them (Cassidy because she's too moral to absolve the girls of their role in the murder and Chugs because she's defending her brother). That's going to tear serious holes in the 'Garrett did everything in cold blood and we had nothing do with it' party line so Jessica drops it out of pragmatism.
Improbable Weapon: The "pimped-out tire iron", used sometimes as a simple three-pronged stabbing-implement, sometimes as a disk-of-death, and at one point as a Killer Yoyo. Gotta give 'em an A for creativity.
Instant Death Bullet: Averted. The shotgun blast doesn't instantly kill Andy, but falling into a flaming pit certainly did.
Jerkass: The great majority of the cast. In fact only Cassidy, Ellie, Maggie and Mrs. Crenshaw seem to be non-jerkasses, and only one of them get killed.
The Killer Becomes the Killed: The main girls find Garret at the scene of Megan's death after the killer calls them, blackmailing them into returning. Suspecting Garret of being the killer due to his being at the original scene of the crime as well as covered in blood and generally acting crazy (in reality he was grieving and cutting himself), they hit him with their car, seemingly killing him... until he seemingly reappears at the end. Sequel Hook?
The Lad-ette: Chugs has elements of this, being hard drinking (hence the nickname), sexually aggressive and having poor personal hygiene ("You taste like vomit", quoth one boy after she sticks her tongue down his throat). It gets to the point her answering machine instructs callers to leave a message after the *BURP*.
Meganekko: Ellie, though she eventually loses her glasses.
Modesty Towel. Subverted when one girl is forced to remove her towel and walk back to her room (on the other side of the unrealistically large dorm building). The girl runs back in the buff gleefully (because she probably had to go get ready for one of the sex parties).