The mystery is: Why is it so hard to make a decent Aliens vs Predator movie? There's no reason for them to be this bad. There are a million potentially awesome ways to combine the two franchises. 20th Century Fox could've picked an AvP fan script from the Internet out of a hat and odds are they would have found something better than what they've used so far. AvP:R -- the second film in the franchise -- is better than the original only because it's rated R and because it doesn't take place in a futuristic pyramid in present-day Antarctica.
Requiem starts with a Queen Alien/Predator hybrid bursting out of the chest of a dead Predator from the first movie and crash landing on earth. Another Predator hears about this, gets upset and flies to Earth to hunt Aliens and disintegrate their corpses (and those of any humans nearby) with blue goo straight out of Harry Potter.
Is the Predator's mission to protect the continuity of the Aliens franchise? Could be. Whatever his purpose, Predator eventually gets into a wrestling match with the Alien/Predator Queen hybrid and then the humans blow everything up in an ending that manages to rip off both The Terminator and Return of the Living Dead.
It's obvious that a movie called Aliens vs. Predator should focus on Aliens and Predators, yet this is the second time humans have gotten the most screen time. I can see people on TV any time I want. If I pay to see a movie called Aliens vs. Predator, I want to see those two things with the humans left out. Or at least make one or two human characters not utterly suck. Also, giving a woman a gun and a kid to take care of doesn't make her Ripley any more than giving a woman a giant, perfectly sculpted man-jaw makes her Sigourney Weaver.
They probably should have avoided anything that would cause people think of Aliens and then wonder why AvP:R is so bad by comparison. For instance, one great thing about Aliens is that it doesn't have any subplots; it just had the one plot that was sufficiently exciting to carry the movie through two and a half hours. AvP:R has a subplot about a guy with girl troubles whose brothers just got out of jail and who is also friends of the newly elected sheriff who lives in the same town as the mom who returned from Iraq and is estranged from her daughter but probably forges a new connection when they see dad getting eaten by Aliens, which have something to do with a Predator who has come to Earth to avenge the death of his brother. Or the death of his boyfriend. Or he just wants to kill more Aliens. I have to guess at Predator's motivation because they all talk in an inscrutable clicking language and have no subtitles.
Released on Christmas Day, AvP:R is the crappiest present of all. 20th Century Fox has sold out the Bedford Falls of everyone's nostalgia for the sci-fi franchises for the Pottersville of a cynical, soulless cash-in. It's exactly like the commercials said, "Whoever wins... we lose." (Rated R)