Fat people. "Rule #1: Cardio." Columbus warns the viewer that being able to outrun zombies is a survival skill and that "the fatties" fell first due to being out of shape, and: "Poor fat bastard."
"I'm in Garland, Texas. It may look like the zombies destroyed it, but that's actually just Garland."
Alternate Character Interpretation: The Big Four are all dealing with the zombie apocalypse and the resulting tragedy in their own way. Tallahassee is all about killing zombies in creative ways so he doesn't have to think about his son being horribly killed. Wichita screws over the guys repeatedly because she doesn't want to deal with all the death. Little Rock is adamant about getting to Pacific Playland so she doesn't have to deal with the fact that everyone she knows is dead, other than Wichita (and Wichita is just trying to make her happy so she has a goal to focus on). And Columbus? Well, Columbus is actually kind of normal. Maybe repressing.
Angst? What Angst?: Columbus really takes accidentally killing Bill Murray in his stride, and Tallahassee seems to get over the death of his idol fairly quickly. Then again, so does Murray. Might be related to surviving a zombie apocalypse.
Big-Lipped Alligator Moment: The "Zombie Kill Of The Week" scene, where an elderly nun casually crushes a zombie under a falling piano. It makes sense out-of-context, however; a TV series was originally planned, in which this scene would have been a Running Gag.
Wichita says that Anaconda was the first R rated film she saw. Anaconda is PG-13.
The "Submarinos" on sale in Mexico are actually Twinkies' knockoffs, Tallahassee will be very disappointed if he finds out. The good news: the real deal it's on sale in every convenience store there too.
Columbus mentions how he's glad that Facebook is no longer relevant in Zombieland. His actor Jesse Eisenberg went on to play Facebook founder Mark Zuckerberg in The Social Network. Even better, Eisenberg wasn't familiar with Facebook at the time and needed the director to explain the joke to him.
For those who recognize the Left 4 Dead influence and have a black sense of humor, people named Bill don't exactly have good luck with zombies, do they?
One-Scene Wonder: Sister Cynthia Knickerbocker, a sweet old lady who drops a piano on a zombie for Zombie Kill of the Week. Six seconds of screentime, and one of the most memorable scenes of the movie.
Periphery Demographic: For a zombie horror-comedy (and so, marketed to a male audience) this movie has a very large female fanbase; good character development for the balanced core cast and the Adorkable male leads probably contributed.
The sisters' first con job plan on Columbus and Tallahassee seemed to hinge on both of them being unable to bring themselves to shoot Little Rock. If Tallahassee were just a bit less compassionate, he might not have hesitated and instead blown Little Rock's brains out before Wichita could stop him.
When Wichita and Little Rock arrive at the Amusement Park, they turn on all the lights and rides, apparently not taking into account that this would attract the zombies.
Bill Murray deciding to sneak up on an armed-and-prepared Columbus by pretending to be a zombie. In his defense, he was stoned.
Columbus has elements of this. He clearly had not much of a life before the zombie outbreak, and even Wichita (who's still a Jerkass at this point) feels sorry for him after she reveals Columbus, Ohio is in ruins and that his parents are either zombies or dead.
Tallahassee, when it's revealed he lost his puppy to the zombies. That's sad enough in itself but when it's revealed it wasn't his puppy, but his SON....