I saw Zombieland on Saturday, and what's so great about the movie was not witty one-liners (though those were funny) and the graphic violence (that was awesome too), instead, what I liked the most about the movie was the formation of a tribe under duress. This movie is more about disparate people working together and bonding as a clan than it is about a zombie apocalypse, much like how Lilo and Stitch is more about family than about aliens, and District 9 is about the self-actualization of the protagonist instead of a pure alien shoot-em-up.
Now I'm not saying that zombies and action aren't prevalent: they are. I wouldn't recommend this movie unless you can handle blood and guts and human cannibalism. Guns and weapons are prevalent, and are used liberally, and with much carnage.
At its core however (and at the center of all good cinema) are the new explorations of universal themes, of which realistic human interaction is a critical component. The motivations of these characters are plausible. The dialogue flows from scene to scene without being forced and telling. I empathize with them, and fear for their safety.
Unlike other movies that exist only to up the gore ante, without a characterization in sight, or a bounty of shallow motivations such as "I don't want to die" that are never fully explored, Zombieland understands that we are complex beasts, even when under attack from the undead.