This is one of the interesting things about the whole Alien franchise that there are alien related movies that fit into different genres. There are horror movies or what ever you want to call them (Alien, Alien3), action movies (aliens), adventure sci-fi (Prometheus), a kind of borderline mix (AC) and then there are the bizarre things that I am not sure what to call them (A4, and AvP).
Keeping this in mind it is not strange that that not all liked AC.
I think that there are aspects of almost every movie that I like even though I can rank them depending on their over-all content and mood. As far as the three groups go I would probably fit into the first one if I had to choose.
One of my complaints about Prometheus (aside from mostly bad characters) is that the Xeno connection was too vague. I see how the Trilobite connects to the Xeno but the Hammerpedes were just lame and I still consider them to be a miscalculation by those that decided to include them in the movie, not very much of a Xeno connection there. This doesn’t mean that we needed to be spoon fed about the answers but I just wish that they could have done the Hammerpede-Xeno connection a bit less vague but not over-explain it. Eventually I can imagine many monsters that are closer related to the Xeno compared to the Hammerpede and the Squid, that would have been good.
They could give us some more clues to the SJ and the Eggs just don’t make it too vague ala Prometheus and not overexplanatory. Fox messed up on this one.
Human characters is number one because if you don’t care about them then it doesn’t matter how good the landscapes look and how interesting monsters that you come up with. If Prometheus would have had worse monsters but better human characters that would still have been a plus because then we (meaning the movie audience in general) would have given a damn about the humans which is what a movie must achieve in my humble opinion. For example, imagine if Macbeth would have had crappy characters, would it have become a classic then? I doubt it and that probably goes for books, movies, what ever.
IRaptus: You forgot Alien 3 in your division list. :D I see that you mention Alien 3 later down in the discussion). That is nice to see that Alien 3 gets some attention.
Joylitt makes a good comment where he or she says that there needs to be some mystery but I would like to add that there shall not be too much that don’t make sense, that is where Prometheus failed.