Why was it so bad? For those that don't know the story, basically a dude wakes up in a hospital 28 days after a pandemic has broken out in England. There really isn't anyone left, and so he sets out from the hospital. Along the way he finds two people who take him in and help him. Then those three become two (one dies) and they move on to find a dad and daughter hold up in building. Once together, they move on to a compound of military people keeping out the infected. I'll stop the story there, as to not ruin it.
What made it so bad is that it was slow. There wasn't much dialog, and when there was dialog I just didn't care for it. My co-workers slam me because I don't hate on "I Am Legend" even though its basically the same movie, with even less dialog. However you can look for that review later. To sum up this junk fest, its a zombie movie, but horribly boring and slow. It is somewhat artsy if you're into that kind of thing. I will say however that the sequel "28 Weeks Later" was much better, and you can read that review in the near future.
My final recommendation - don't watch this movie!