Alien Vs Predator is based on two of the most popular sci-fi/horror franchases in the history of film. Fans of both series, in both film, book and comic book culture, were very much looking forward to a film being made involving these legendary characters. Finally Paul W. S. Anderson made such one.
And it sucked.
Background on the film: The movie takes place in Antartica in the present day. A heat source has been discovered by Weyland Industries (which is commonly referred to as 'The Company' in the Alien films) which ends up being an ancient. In a race against other companies looking to discover what this is, Bishop Weyland compiles a team of explorers, researchers and cave divers.
When they begin exploring the pyramid they discovered several cultures developed it (Egyptian, Incan, etc). Suddenlly one of the team members inadvertanly sets off the pyramid to begin the process that has been done every few hundred years: the breeding of Aliens so the Predators can fight them and therefore, prove themselves to be true warriors. From there it all goes downhill.
It wasn't so much so that script or story sucked. It was the fact that this movie was degraded down to a PG-13 rating. When both franchises have always been rated R. Because of this degrading it has affected the plot and much of the action sequences of the film in order to advertise it to younger audiences.
What also affected this film is that Anderson even went out of his way to mess with the design of the reputed adversaries themselves. The Predators looked like oversized, steroid-laced things. Which in watching the film, you can obviously tell that the actors that wore these costumes had a hard time walking, much less running and fighting. The Aliens, which were based on the artwork of H. R. Giger, were made too bulky. They were supposed to be fluid in their movement as they have in previous films. But like their Predator counterparts, they can hardly move.
The ending is an obvious hint that a sequel will be made - and was - and thankfully was done in a much better fashion than what Anderson did.