Alien Vs. Predator
The main problem with Alien vs. Predator is that it was hard to define what kind of movie it was. Technically, it would fall under the science fiction category. (Two alien species trying to kill each other.) But that doesn't work.
When you're through watching a science fiction movie, you want to be able to say, Wow, could that really happen? It should make you think about life here on a grander scale. AvP didn't do that. Even the teaser at the end wasn't unexpected.
Alien vs. Predator as an action movie. Yep, this fits the bill. Instead of Van Dam and Bruce Willis beating the crap out of each other, you have the aliens. They fight. One wins. And neither gets the girl.
AvP as a romantic comedy? Ummmmmm....
That leaves (well, I'm leaving a lot out, mind you!) Big Monster Movie.
You know, Godzilla. King Kong. Two big monsters who go around and destroy things while trying to kill each other. (And, as luck would have it, that's the plot to AvP!) Nothing too complex. No big issues to resolve. No need to think. Just plain destructive fun.