Just as an example, consider the issue of illegal immigrants, which Cuaron hammers like he's Hephaestus on amphetamines. It seems pretty likely to me that if fertility hits the skids, then countries will look to *increase* immigration, rather than ban it, and that a lively international competition for youths will result. Instead, Cuaron's London sees illegal immigrants herded into cages, in public areas, and then shipped off to concentration camps. (James's novel, from what I gather, posited that immigration would go up, but not grantings of citizenship, and the immigrants would be used as a segregated labor force. Aged resident aliens would be turned out to save the government from having to pay for their retirement and medical care. To me, that's a much more believable dark scenario than Cuaron's, not because it's any less evil but because it fits the situation much better.)
There are other little things like that, things that don't quite fit. For example, there's a lot of graffiti. In one scene, Clive Owen and Julianne Moore walk through some back alleys, and you see wall to wall graffiti everywhere. Except, of course, this is a place with few kids; the youngest people in the world are eighteen years old, so it stands to reason you've got a population bulge at the other end. There are going to be a lot more people cleaning up the graffiti than painting it. For Cuaron, graffiti's a way to establish, visually, resistance to the dictatorial regime he posits for Britain, but it doesn't fit the world he's playing in. Likewise, when Owen and Moore walk past a fallen stroller, it's supposed to symbolize the fate of the human race. Except the stroller has good cloth on it, when by all rights it should have fallen to bits years ago.
Political and conceptual problems aside, the film works very well as a thriller. You know all those fancy one-shot takes Joss Whedon likes to do on occasion, where the camera weaves in and out and around and never cuts away? Cuaron sends the boy to school. There is steadicam work in CHILDREN OF MEN that you will not believe. The level of choreography, of detail, of orchestration is amazing, and sometimes baffling; one attack on a car containing our heroes is so magnificently done that you follow the dialogue, the action, the escalating tension, gags including practical effects and small-scale but impressively performed stunts, and then you realize that the camera is literally spinning around inside the car and you *have no clue* how it was done -- where the hell did they put the cameraman? It's almost a little too showy; the one-take gimmick really screams "Look at me, ma! I'm filmmaking!" but Cuaron makes the best use of it that anybody ever has since Orson Welles did TOUCH OF EVIL, and he's very good at using it in moments where the stakes are especially high for the characters, so it drives home the notion that *they really can't screw up.* It's out of those scenes that you wonder what Cuaron is thinking.
Final example of said dumb stuff: near the end of the movie, two characters are in a rowboat off the coast of London, where they head for a specific buoy in order to make a previously-arranged rendezvous with a boat. There is no guarantee that the boat will be there. Or, since they're late for the rendezvous, that the boat was there, or will come back. They find the spot, and there they wait. For hours, maybe a day, they don't know.
Except they don't tie a line to the buoy.
They sit there, and talk, and wait, and it's a little thing in the scheme, but I had to grit my teeth to keep from yelling, "TIE IT UP, you stupid prats, haven't you heard that water has these things called CURRENTS?! Tides? Do you want to MAKE THE FRIGGING BOAT?!!?"
So CHILDREN OF MEN has a director who does a great job of directing the action, but not so much the story. If you can turn off your brain at the right times, it's a well-made film. Just don't make the mistake of thinking it really is as smart as it thinks it is, because it's not so much smart as smug. As an action flick, though? Boy howdy.