My thoughts parallel James Berardinelli's:
And, so far, among the 14 films I've seen in 2007, there haven't been any "bad" films as much as just sequels with a lack of inspiration, or lack of a good story. (Spider-Man 3 and Shrek the Third were #11 and #13, respectively, beforehand. And they still are. At least Shrek the Third could have been fresh if it was the original film; this film would have garnered more points for its look, but since it's duplicating a look from a much better film, and regardless of look, suffered from characterization (making it the original doesn't take away the weaknesses of the writing, which just causes it to suffer more in the wake of its predecessor), it's gonna have to hit #14 of 14 so far.
28 Days Later, while not terribly original, was suspenseful and involving. 28 Weeks Later is neither. The characters aren't as sympathetic or interesting. The kids are generic and the script doesn't care much about the adults. Robert Carlyle, Catherine McCormack, and Rose Byrne are criminally underused. Compare them to Cillian Murphy, Naomie Harris, and Brendan Gleeson from the first film, all of whom inhabited better developed and more sympathetic personalities. Tension in horror movies results from viewers caring about what happens to characters. The audience's connection to the protagonists of 28 Days Later made it a compelling experience. The lack of such a connection in 28 Weeks Later reduces this to a number of sequences characterized by shock moments, frenetic (and often chaotic) action, and stylized gore - all without suspense.