Probably the concept of humans coming together on equal terms to fight a common, undeniably evil, enemy. They typically eliminate most complications of real life (Jobs, money, car repairs, whatever) and replaces them with issues that are important to every survivor, instead of just individuals. They commonly work together to build defenses, gather food and supplies, and set up new societies. Unlike in real, modern times, where most people are left to fend for themselves, and are bound by laws, responsibilities and the shades of grey that define "good" and "bad."
The reason I'd like a zombie apocalypse is because I could beat and shoot and stab people without feeling guilty because they're not really people anymore.