First of all the movie was pretty gory, so if that’s your thing it delivered. But the main reason the movie appeals to so many people is its social commentary.
For starters the zombies aren’t the biggest threat to humanity, humans are. All throughout the movie the humans either fight amongst themselves or get overly cocky which leads to their own demise. Remember, Roger and Peter run through the mall and around zombies with little trouble, and the only time the zombies become a threat is when people stop working together and fighting amongst themselves.
The guys at the TV station are arguing about whether or not zombies are cannibals, when really, who cares? They should be arguing about how best to kill them. The one SWAT guy gets so cocky about being able to run around the zombies that he allows himself to be bit thus ensuring his own death. The bikers are overly confident and allow themselves to be taken down by the zombies even though they have weapons and vehicles. And Flyboy screws his whole group by deciding that he’s going to fight the bikers for the mall, which is pointless because they could just close it up again after they left.
Flyboy’s dumbassness brings me to the other major commentary of the movie, which is humanity’s over-emphasis on material things. The whole point of the shopping montages and their robbing the bank is to set up the heroes’ realization of how worthless material goods are if there aren’t other people to show them off to. You notice that after Roger dies they all become despondent, not only over Roger’s death, but also because even though they are holed up in a mall with every material good they could want they are still unhappy.
Then you have the bikers who fight for the mall thus setting up its loss when if they had cooperated there was plenty of goods for all of them. Flyboy decides to fight for the mall against the bikers even though there were more than enough goods for the heroes and the bikers, which of course leads to his own death. And then there are the zombies, who flock to the mall because of some vague memory they have from when they were alive. That right there is the biggest anti-consumerism message of the whole film: What would the dead do if they came back to life? They’d all go shopping.
And of course, another reason the movie is so well loved is the zombie whose head explodes from a shotgun blast at the beginning of the film. That was fucking cool.