Ax-Crazy: Candy and Brit come across as such in the latter half of the movie, considering they have the guts to stick loaded guns in the mouth of a gangster who was seducing them, and casually discuss offing him and taking his stuff. Even in the first half of the movie, their go-to plan for getting money - robbing a local restaurant and threatening to kill the people inside - screams that they are not stable, and Faith's church-buddies warn her that they are evil.
Bastard Girlfriend: Alien gets lucky with two hot young blondes... good for him, right? Well, they put loaded guns in his mouth, further humiliating him by having a cheerful little discussion with each other over his head... this discussion being about whether or not they should just kill him and take all his stuff. His reaction is to fall in love with them. (Or pretend to - but he never try to take revenge.)
Billing Displacement: On the theatrical poster, Selena Gomez is the first billed actor, despite her character leaving the film halfway through. Justified though due to Gomez's character being the main focus of the film until James Franco arrives.
Contrast Montage: Much of both the film's humour and drama comes from juxtaposing naive idealism with cynical debauchery and, ultimately, bloody violence. In between drug fuelled parties and violent crimes, the protagonists are seen watching My Little Pony: Friendship Is Magic and singing Britney Spears songs. On a similar note, the bloody climactic scene features two characters wearing pink balaclavas with unicorns emblazoned on them.
Despair Event Horizon: The reason as to why some characters were Put On The Bus. Faith's comes after being bailed out by Alien and not feeling comfortable in his presence, while Cotty getting shot is the kick in the ass she needs to leave.
Earn Your Happy Ending: From a very perverse perspective. Faith and Cotty become disillusioned and cynical after their experiences, and end up going home. The other two, however, stay on and end up rich after a successful killing spree. For them, the dream of "spring break forever" has come true.
Fanservice: The girls only wear their bikinis once they reach Florida unless they decide to be Put On The Bus and Alien is almost always seen without his shirt...
Fan Disservice: ...on the other hand, while there are plenty of shirtless extras (of both genders), it never feels quite right, what with the excessive slow-motion Jiggle Physics and unflattering camera angles. Also, the only time we get to see Cotty's breasts is when she's having a Shower of Angst and covered in blood, having just been shot in the arm a few hours prior.
Also, despite all the advance publicity, Selena Gomez's character never gets naked nor partakes in any of the film's sex scenes.
Foil: The two Power Trios - Alien, Brit and Candy vs. Big Archie and his two unnamed companions. Alien seems to genuinely love (or at least put in the effort to pretend to love) his two female friends, caring for them and helping them, while their Three-Way Sex is passionate and engaging for all involved. Big Archie, by contrast, treats his companions like sexual objects and doesn't even bother to pay attention to them. The contrast is highlighted in the final shootout, when Archie's girls are busy feeling each other up in the shower while Brit and Candy rush in and kill Archie.
An interesting auditory example. The movie is broken into roughly four segments, and the transition between each is a specific event that has a distinctive sound. The sound is worked into the edits and shot changes in the scenes leading up to their events. Specifically, sounds of crashing and hammering leads up to the fast food robbery which heralds going to Florida, sounds of the jail cell door leads up to the arrest which moves from typical spring break to weird spring break, and sounds of gun fire leads up to Cotty being shot.
When the girls try to build up their courage prior to the Chicken Shack robbery, one of them says "Just pretend it's a video game." The final shootout itself is very video game-like in its structure.
The girls' reenactment of the bank robbery in the parking lot shows that the lot of them are a little too dedicated to abusing Faith, who plays the role of one of the Chicken Shack customers. This helps show us how easily the three girls slip into Alien's violent power fantasies.
Gender-Blender Name: Many people not familiar with Harmony Korine's work were surprised to find out that he's actually a man.
Hidden Depths: Alien playing Everytime by Britney Spears flawlessly on the piano shows us that he likely had formal training at some point though he could have just learned the song to impress girls like the gang.
Improbable Aiming Skills: A very egregious example since Brit and Candy killed everyone on the gang compound using sub-machine guns held with one hand, despite the movie never having shown them shooting anyone before.