I'm not picky with regard to my science fiction. Like, I really un-ironically enjoy a lot of campy science fiction, but I think Promethius actually did a terrible job suspending disbelief. In my opinion, they massively violated cinema kosher by mixing gravity and gaiety, both in HUGE amounts. When you say things at the outset like
"I have spent my entire lifetime contemplating the questions: Where do we come from? What is our purpose? What happens when we die? And I have finally found two people who convinced me they're on the verge of answering them." you are going to invariably short-circuit the part of my brain that would otherwise be okay with all of the following things:
- astroarcheologists on the most important mission in the entire sphere of human history touch, electrify, inhale, smoke, and screw everything they encounter.
- Von Daniken style paleo-aliens that intervened at many points in human history, enough to leave maps all over the world, but apparently hate us and consider us as dust and don't mind killing us for fun...also, apparently they left us little maps to their secret bioweapons research lab.
- totally implausible biology on both the micro and macro scale
- lack of discernible goals or motivations in all levels of antagonists, from weyland (why did he lie about coming?), to the android (why did he infect that one guy?), to the space jockeys (why did they create humans... then direct humans to their bioweapons facility...then kill them...then try to destroy earth?)
it's not simply that they do this at the beginning and then things return to normal. They lace the whole movie with really important conversations about teleology and mortality and stuff, which forces me to use parts of my forebrain usually inactive when watching things like "Predators" or "Pacific Rim"
I know this is a bit of necroing but I figured I would post a reply here instead of creating a new thread and referring to this. There is apparently a lot of stuff that Ridley Scott came up with for explaining things, like why there is the head ripping, that didn't make it into the movie. I am hoping for a director's cut or something similar to eventually come out and put in all the backstory that can be found online that makes this a much better movie.
This to me is a HUGE warning sign that something has gone wrong. I recall having conversations regarding both "Halo" and the Star Wars movies to the same effect: any reasonably creative person, if they sit down and think on it long enough, can connect things into a coherent explanation* for why weird stuff happens. The question is whether things happen for a plausible reason
for your audience, that is, is the explanation contained in the movie...or, if it isn't...is there a good reason for the lack of reason.
*
here's mine: The jockeys created humans because they wanted a prototype for a grander creation. Earth is Mk. I and Mk. II was being built on that new planet. We were supposed to be exterminated by a later crew but in building mk. II, something went horribly wrong and the space jockeys died, never returning to earth again. The cave paintings were elements in the human collective unconcious which also prompted the expedition, hence destroying the implausibility of going to this planet on a hunch...humans are programmed to want to go to that planet. Weyland pretended not to come because his plan was to, behind the scenes, kill every member of the ship in order to make contact ALONE with his android, which explains the android's malice. Headrip happens because the jockeys know Mk. I humans are Mk. II weapons, like the garden of eden allegory (deeply implanted in our subconscious), they know that now that we have knowledge the only thing that prevents us from becoming exactly like them is eternal life. Weyland's quest for eternal life is their worst fear. 
