Abusive Parents: Bo and Vincent's parents strapped one of their children into his high chair if he was misbehaving and the mother is shown hitting him in the face. Nick and Carly's parents were emotionally abusive to Nick during his teenage years, frequently comparing him to Carly and eventually leading him to think of himself as The Unfavourite.
Annoying Arrows: Averted. One of the killers brothers is shot and impaled with two arrows. He slowly (and very painfully) pulls out one from under his arm (which releases a lot of blood, and he promptly bandages it) and then tries to remove the one from the side of his chest, but the pain makes him vomit. Rather than risk passing out and bleeding to death he just cuts off the end of the arrow, leaving the rest in his chest. While one could argue that he should still have died from these injuries, Carly beats him to death with a baseball bat later anyway.
Backup Twin: At the end of movie Bo and Vincent are revealed to have a third brother. He's not a triplet though, evidenced by the beginning scene.
Nick. Not once, not twice, but three times encountering and singlehandedly beating the hell right out of the main villain, including shooting him with a crossbow. Considering that one of these encounters involves the villain ambushing him with a knife, and the other has him effectively outnumbered in a slowly-melting house...
Carly, in a way, qualifies. Having the self control to calm down and use her own blood after having her finger cut off to open her recently glued mouth so she can warn her brother was pretty impressive.
Beauty Is Never Tarnished: Averted for the females as Carly gets her finger cut off and her lips glued together meaning she has to pull them apart and spends the rest of the film with a bloody mouth. Even Paige gets stabbed in the ankle. However Nick manages to go through the whole film with only just a little bit of dirt on himself.
Big Brother Instinct: Nick towards Carly, as revealed when the killers attack. She more than returns the favour in the climax.
Camera Fiend: Dalton, who is quite fond of filming the couples making out.
Evil Twin: Discussed by Carly. She said Nick often got a hard time from their parents, leading to him thinking of himself as the evil twin. Bo is Vincent's Evil Twin.
Fanservice: Nick removes his shirt for Carly to wear which allows viewers to see Chad Michael Murray shirtless and doubles by placing Elisha Cuthbert in a tank top for the rest of the movie. Oh and Paris Hilton's striptease of course.
Rule of Sexy: Why, exactly, does Paige feel the need to strip down to her lingerie and do a striptease right before breaking the news that she's pregnant to her boyfriend? The director probably doesn't know either. But, dammit, they paid for Paris Hilton's bod, and they were gonna show it off!
They were about to have sex, but she stopped because she wanted to actually talk instead of fool around.
Shirtless Scene: Blake gets two. Nick gets one where he gives Carly his shirt to wear.