I don't think more dwelling on the kid's death would have helped the movie. I already felt like they used it as a device to fit Owen's character into a neat little box: He used to be an Idealist, but suffered a Traumatic Event, and now is Cynical. Maybe it works better in the book, but I can't help thinking that in the movie, you'd just wind up with a series of flashbacks like the ones Tom Cruise had in The Minority Report.
Also, there's that slightly cheesy sci-fi tendency to throw in some clever extrapolations of the main plot device. What if there were no more babies and people were dying? They'd market a suicide drug, with the same comforting style as sleeping pill commercials! I kind of felt like, I dunno, they were a bit too pleased with themselves for coming up with that.
Compared to 28 Days Later, which I love pretty unconditionally, where they just let the characters interact with each other, and you get a sense of the impact through that.
But yeah, the cinematography was gorgeous, Clive Owen's great, and the pacing of the chases and lulls was really well done. I've written too much about what I didn't like, (at least I'm not carping about Y Tu Mama Tambien yet!) but I really enjoyed it. As an action movie, it's right up there.