I haven't seen the movie and your review semi-confirms the suspicions that made me not want to see it.
I'm always glad when SF takes on Big Ideas, but if you are going to lecure me I expect the same level of care in mechanics as in message.
This means aliens burned by water do not invade a planet with rain and oceans and squirtguns without at least putting on some clothing. Children of Men suffered from similar logical flaws, as did the sequel to 28 Days Later.
Aliens in posession of superior technology should use it to their advantage, or failing that, a minimally competant government should deliver a whole assload of buttermilk (sorry) catfood to completely disarm the aliens. Maybe the filmmakers address this point with a global catfood shortage, fair enough.
The last film that engendered this level of loyalty was Equilibrium: a steamy load of allegory made outside Hollywood that was, if nothing else, at last as sloppy in its message as in its mechanics.
Maybe District 9 is better than Equilibrium, but if the commercials suggests WTF-level plot holes it will take more than a creamy review from Knowles to get me in the theater.
I'm glad to read one post that allows that I may not be completely wasting my life between now and whenever I get the movie from Netflix.