Alien or no alien, it's clear from the footage so far that the film is focused around elements of horror. It's a science fiction horror mystery like the original Alien film. People are running in fear, screaming, and there are creepy aliens or mutants or whatever they turn out to be. It's like the original Alien formula complete with robots and sexy lead actresses, Giger designs, but with a bigger budget. All the best elements from the original just ramped up. It's a way to reboot the series. No complaints here, but ultimately its still the same formula as the original.
My point is, Prometheus was being spun as if this was a totally different film that just happened to be set in the Alien universe, but this is no whimsical space adventure or some other type of film - it is the genre of the original Alien film and as in the original, most of the crew will be killed in some terrifying manner by some unusual life form.