Its a much more involved story than its seems like it would be. Its definately not just a Night of the Living Dead, go hide in the farmhouse scare flick. For as intense and creepy as it is, there is a nice amount of character development. There is actually not a bunch of human eating type stuff you might expect of it. The infected people are some of the creepiest characters I've seen in a movie, much more realistic and frightening that your typical comedic undead.
What makes it such a great movie is the realistic feel though. You feel like youre there with these people, like this really happened and someone happened to be there with a video camera. It's not Blair Witch real (as in motion sickness camera movements), but it does kind of have a documentary, non-invasive filming feel to it. Add to that a completely plausible story, something not completely zany and out in left field. It does make a lot of sense, and in an age of biological weapons, new diseases, etc, it has a very realistic feel. You can almost become convinced that, hey, something like this could happen, somewhere, somehow. Its a very reasonable story.
Another little thing that is nice about it is the fact that it literally jumps right into the movie. No beginning credits, etc. As soon as the Fox Searchlight music ends and the logo fades, the movie starts. If you didn't know the name of the movie, you might actually not figure out what it was called. The make the title part of the story, just as a piece of progression. The story begins, goes through a brief story of how it all began, "28 days later..." fades in, and the next scene starts.
Hey, and Brendan Gleeson is in it, a fantastic actor that doesnt get nearly the credit he deserves.
Im really starting to become a fan of Fox Searchlight pictures, they are really tossing some great flicks out there for people to see (One Hour Photo was fantastic, and the upcoming Garage Days looks like it could be decent).