Also Curt, who is tailor-made to look like a jock, turns out to be a sociology major on a full academic scholarship. When he tells Dana not to waste time reading a particular book, he suggests a better book for that class. It's completely unexpected, but sets the tone nicely for a movie that's all about subverting audience expectations.
Also in the opening, Curt's and Jules' PSA parody.
(Jules is holding complex textbooks)
Curt: Where did you learn about this stuff?
Jules: From you, okay? I learned it from watching you!
The evil unicorn. Just... the evil unicorn. The BGM helps.
The clown, disturbing as it may have been.
The stoner character Marty remaining "sober" while everyone else succumbs to the mind-control chemicals.
Patience sticking her head out of the elevator and looking at the blood-soaked hallway with an "Aw gee, I missed all the fun" expression on her face.
And the second team of security sent to the elevator room. They run in, blood everywhere and silence. Zombies look up. A beat. *ding!*
On the subject of zombies: After the first attack by them, Dana insists on going to search for Jules. She opens the cabin door...only to be confronted with Matthew Buckner, who casually tosses her Jules' severed head.
Also on the subject of the redneck zombie torture family, the scene at the cabin, right after Jules' death, but before the head tossing, Curt's panicked declaration to run to Marty, before clotheslining Patience...was hilarious.
"Good work, zombie arm."
"Cleanse them. Cleanse the world of their ignorance and sin. Bathe them in the crimson of — am I on speakerphone?"
Even funnier is when Mordecai tries another evil-sounding rant, pauses, and says, "I'm still on speakerphone, aren't I?" While Hadley unconvincingly tries to deny it, Sitterson is laughing hysterically.
"That is just rude. I don't know who's in the room."
When Jules and Curt are about to have sex, we see all of the male scientists ogling her. Hadley and Sitterson send them all away...so they can watch themselves.
During the party, Ronald is heard telling someone "I'm the intern!" Later, during the System Purge when the monitors showing the chaos are being viewed, Ronald is shown holding a sign up to the camera reading "Help me! A dragonbat has my scent. I'm the intern."
According to the Cabin in the Woods Official Visual Companion, Joss Whedon mentioned that some things will never be known. Such as "How did Ronald the Intern survive the end of the world and eventually end up ruling the ancient gods?"
When they find the artifact:
Marty: Okay, I'm drawing a line in the fucking sand here, do NOT read the Latin.
Marty: What are you saying? What do you want? You think I'm a puppet, huh? You think I'm going to do a little puppet dance? I'm the boss of my own brain so give it up! *beat* I'm going for a walk.
This bit after Dana escapes the sunken van:
Hadley: I'm actually rooting for this girl. You gotta admire her heart— (some other guys bring in a bottle of tequila) TEQUILA IS MY LADYYYYY!
In the DVD commentary, Joss Whedon gets a pretty funny line of Self-Deprecation/Hypocritical Humor. He's explaining how they didn't want to make all the monsters shout outs to popular horror icons like Freddy and Jason because "If you took a bunch of iconic characters who had already been in a movie, and then just made a movie with all of them together, you'd be a disgusting hack. *beat* Oh God! We're gonna cut that out, right?"
Also, if you watch the DVD commentary... stick around through the credits.
Marty finding the hidden camera in his bedroom:
Marty: I'm on a reality TV show...(grins for a moment before dropping it and realizing)...my parents are gonna think I'm such a burnout.
During the climactic monster rampage, a zombie clearly grabs an employee and throws him over the railing into the waiting jaws of the giant snake. This one scene, coupled with the lack of even a hint of conflict among the monsters themselves forces us to conclude that they're all friends. A million and one mental images follow.