> Random spoiler filled thoughts!
For me some of the most apparent themes were alienation, empathy and emotion, sexuality, the human condition in general, the hunter/prey relationship, role reversal, cultural evolution as a memetic outbreak, psychopathy as armor, ... and that might only be scratching the surface.
Role reversal seems to me to be a big one. The men are the prey, lured to their death by emotions like lust and a longing for connection. The woman is the hunter devoid of any real feelings, she doesn't even kill for her own consumption but is neutral and indifferent.
As the film progresses, Johansson seems to be infected with empathy and emotions or perhaps even the entire human condition. As her ability to feel emotion grows, she grows more vulnerable, ultimately she is flooded by them and the hunter becomes the prey. She flees the controlled environment of the city and perhaps hopes to find refuge in isolation but the more unpredictable nature of humans away from any form of control by their peers means that the forest she ends up in becomes a chaotic and dangerous place.
It's a rather bleak picture but the end sees the reversal reversed or perhaps a return to "normal" where the woman is the one who is powerless and abused. Because she allowed herself to feel, she got a crack in her armor. We sometimes talk about emotional armor, I think this film hints at the opposite, emotions might make life more interesting but they can also be dangerous and make you weak while a lack of them brings with it a position of power. She was the perfect drone, an ideal employee as long as she was disconnected from it all. The original Johansson cried when she was stripped of her "skin", the alien Johansson didn't but perhaps only because she couldn't? I got the feeling that it was her emotional awakening that ended up undoing her.
I'd draw a parallel with how the aliens in War of the Worlds were in the end killed by a simple biological virus (actually bacteria if you want to be precise). Perhaps all these mysterious invaders will similarly ultimately succumb to the virus of mind? Perhaps just like certain genes can be dominant and recessive, so too the human condition is dominant and once introduced to it, there is no going back to a life without? We have no way of knowing what the aliens were like before first contact. They appear to be telepathic, maybe they were used to operating like drones without any real ego, doing everything they do in service of the hive? Before being infected with emotion, Johansson almost seemed stuck in a loop, acting out a very predictable pattern. I am reminded of bees who operate on very basic rules (mainly follow the chemical). A bee is thrown into chaos if she loses the hive and thus the chemical trails that guide her. She only started acting strange (read: more human) after experiencing certain interactions outside of her capture and kill routine, like being picked up after having fallen on the street. It's also worth noting that we hear them mechanically learn our language during the first few minutes. Just like a disease spreads through an animal carrier, so our language is the carrier of thought. If we are the first time they encounter a species made up of individuals that uses soundwaves to transmit verbal communication, then perhaps they didn't know just how much of an influence it really has on the make-up of the brain, being unaware that with repeated use and exposure it will rewire the brain? I think they saw it as nothing more than a tool, a means to an end. They just needed to be able to lure people into their venus flytrap but ultimately they turned out to be unable to resist everything that comes with it.
Hell, just Think of the recent facebook experiment in which user's emotions could be influenced positively or negatively solely by increasing/decreasing the amount of positive/negative words they saw in their stream. There's no getting away from that. You can't live in our environment and not experience at least a little bit of what it means to be human. We have our emotions toyed with all the time. A scent can bring a tear to your eye, the sound of the ocean can bring back memories, even drinking a cup of tea can alter your mood. At some point in the film she even starts listening to music, tapping her finger to the beat. That's probably the point of no return because as we all know, music has a way of getting to you and working itself a way into your brain.
Then again, it's just one perspective. Depending on your point of view of for example the motorcyclists, whether they are her equals, her servants or her handlers, I guess you could arrive at something completely different. When they set out after her, are they trying to save her from us? Are they pissed off that she let one of her victims go? Perhaps they don't care much about anything but just want to clean things up because they don't want to be discovered?
I guess the surreal liquid trap functioned as some sort of freezer, keeping the earthlings fresh until someone new of their own kind arrived? That moment where those 2 guys touch each other as one is about to get his meat sucked out of him... chills. ><
I wonder if there's any meaning behind Johansson getting her body/clothes handed to her by the biker who picked it up from a field. They somehow duplicated the skin on that one without having to lure her into the meat remover. Perhaps it hints at the process not working on woman? Maybe it's saying that men are more sex crazed, making it easier to override their thinking by just wiggling some nice bits of flesh in front of them while women might be able to keep their head clear enough to realize that following a stranger into a seemingly abandoned and decrepit building might not be the best idea?
Some of the men also seemed to feel a certain apprehension, especially the man suffering from neurofibromatosis (unfortunately those weren't fx :'(. It probably played a part in why she let him go.
Maybe it does work on woman but something similar happened to the original Johansson. Perhaps the alien that caught her succumbed to empathy as well, feeling sorry for her, letting her escape, leaving the bikers to clean up the mess like they did with the man Johansson set free or perhaps the beginning shouldn't be taken at face value at all but instead as an ouroboros like metaphor? As alien Johansson strips her human self naked, the human self is powerless and was most likely disposed of afterwards, very similar to alien Johansson getting stripped and being disposed of during the films finale. A repeating cycle of people who feels so above others that they treat them like bugs? Now that I think of it, the forester drenching her in gasoline, that's how you'd kill an anthill.

Yeah I am reaching here. ^^
Also, fuck that scene with the baby on the beach. I can't stop thinking about it. When the biker returns there at night to clean up the tent and the baby is still sitting there, crying... I am still getting goosebumps from that but not in a good way.
Another peculiar things about this film is how I almost immediately felt that something about the people in it was incredibly off... Which is really rather weird considering they are actual real people who don't know they are being filmed. Have we become so used to Hollywood acting that somehow seeing the real thing on the big screen feels truly alien? We might think that acting has gotten more natural since the 50s but really, current acting is just a different style only less formal but still as far removed from genuine interaction as the 50s style was. 
I'll probably be watching it again this Friday so maybe I'll have some more to add afterwards. It goes without saying that I think it's good. Real good. Like a wise woman once said; It's the questions that drive us. Films that ask them instead of answering them have always been my preferred cinefood and although this film is in some ways actually rather straightforward it does leave a lot of room to dig into for those who are into that sort of thing.