This was another I picked up last night and watched with all three boys. The best part about it was the acting by Jennifer Connelly and Keanu Reeves, who I thought really did well (as did John Cleese isn a small but well-written and very well handled part). The story was just a bit thin, and the environmental angle was a bit heavy-handed.
The one thing that I thought would have made it more plausible was playing up one thing Keanu said - that there are only a very small number of planets capable of supporting advanced life, and that was why the wholesale destruction of human civilization was justified. Not destroying humanity would have cost future civilizations the chance to be born, so it wasn't simply a matter of weeding out a race with flaws to satisfy an alien compulsive-retentive obsession, but instead a decision to sacrifice the good of the few for the good of the many (as Mr. Spock once put it). More could have been done to show the conflict in the aliens between their desire to preserve new life forms and new civilizations (there I go again) and their desire to preserve a unique environment for the many races that really needed it. In this context, humanity's ability to change (which I think was well-handled through Connelly's stepson's change, and the way she illustrated emotionally what Cleese told her she needed to do) would not have been enough - we had to show not just that we could change but that we deserved the right to keep our place ahead of other competing races.
As it was, it was too easy to simply say that the aliens were bad for wanting to take something away that was ours - or at the very least more ours than theirs. All we had to do was show Klaatu that we could change, and he called everything off. And that didn't provide much dramatic tension to me, making the ending way too pat. I thought it would have been better had we, for example, had to show to an existing alternative race that we deserved to keep our world. That would have introduced some overtones of Independence Day, where humans had to fight to defeat precisely the substitution hinted at here, but it would have produced more draamatic tension. As it was, all I could think of was that the aliens had no right to tell us we couldn't do as we please, even as unwise as we were being. Had we had to justify our survival over an equally deserving race needing a planet - that would have been more of a nail-biter.