If I may...a repost form the other thread...
My take on the whole thing...the aliens were probing the earth, as mentioned by the army recruiter. Maybe they had our planet pegged for a possible invasion, and after their unsuccessful preliminary raid, decided to move on due to the toxic environment. Near the end, the guy on the radio mentions that humans were possibly the direct targets, but really anything that the characters mention about the aliens is conjecture. I don't remember him saying anything about food.
The low-tech nature of the visitors themselves didn't really bother me. If you have amazing agility, skin like a chameleon and natural poison (or were they implants?), I would imagine gadgets would be less important to you. As Furn mentioned, their tech may be completely different from our own. I agree that it was a bit absurd that these aliens, who seem to have a strength at least comparable to our own, couldn't get through a barricaded door. The real question I guess would be how bad do I want to get through that door? I might be able to reach into the burrow of a wild badger easy enough...doesn't mean I'm going to do it, especially if shes protecting her young. I might be more inclined to go after something easier.
In the end it was faith that was the real focus here. The news reports were spartan, telling you that earth is being invaded by martians in five words or less. There were no scenes of panic in the streets. Even in the final showdown, there was more screen time on Mel and his dying son - his brothers fight with the creature in the livingroom was almost an afterthought, seen briefly through the window, or reflected in the TV. Not to mention that the struggle was over in seconds. The aliens were a means to an end.
Added comments: it was mentioned how you often see the aliens through reflections or near reflections...like the knife and the tv screen. Or through filters or lenses...the tv images, through the window, through the glass of water..
Shyalaman likes to put a gimmick like this in all his movies. In 6th Sense, he uses the color red in any scene with the ghosts or anything influenced by ghosts. In Unbreakable, any major event or revelation regarding Bruce's power is hinted with something upside down: his son watching TV upside down when the breaking news of the train wreck comes on. You see Bruce's face upside down I think when he's pumping iron. Upsidedown comic book...stranger with the gun...seen upsidedown.