AndrewC: Yes, but part of me thinks it's a bit too easy; the whole scene felt a bit too ritualistic, though I can see it as a sacrifice (that theme seems to be recurring all over the movie) in order to give life.
DodoGeo: It's really no wonder it's so portrayed as religions in general tackle the subject of creating life and life itself is considered sacred.
Another thing that I don't get is the murals found in every culture on Earth, judging by those they returned multiple times and influenced the development of man.
Kind of a long term project just to destroy it everything.
I think there might be different "factions" of aliens, some which want to keep us alive (think Prometheus the god) vs others who want us dead (the gods which punished Prometheus for giving mankind fire); those who considered the experiment (us) successful kept visiting in order to keep track of things, the others were waiting for the perfect moment to destroy us.
Another reason might be David; as in, the ships in Prometheus weren't really destined for Earth, it's just David that set the course during that hologram. Also we don't know what David told the Engineer when he woke up: just imagine waking from a long sleep, being still groggy, seeing your "kids" at your doorsteps and not only are they intelligent and evolved, but also cruel (yelling and hitting at the girl) and trying to take over your place (creating life of their own in the form of David) - I for one wouldn't want such a kid as a heritage.
This last part also ties somewhat nicely to when David says "we all want to see our parents dead": the founder of the trip (sorry, have a problem remembering names in general) wanted the secret to the eternal/extremely long life of the engineers while always punishing David for being immortal and not having a soul. For all we know David could've told the Engineer "look, they made me immortal and spit on me yet they want to be immortal themselves, what would this mean for you?"
bevinator: the 3d effects were purdy, but I don't think it was particularly memorable.
That's one of the things I love and where the movie really shines for me; being actually filmed in 3D instead of being done in post-production meant that the 3D effects felt natural and nothing over the top which usually makes my head hurt like mad and unable to actually focus on the movie.
Watched it in Imax 3D and it was wonderful because the 3D effect wasn't in your face at all times, just where 3D would make sense (like people in different planes in a shot).