Just a little bit of originality in a horror movie can go a long way, especially because we're all so used to getting hit with the same handful of different "horror types" over and over again. Vampire movies, psycho-killer movies, werewolf movies, body-snatcher movies, zombie movies, etc.
So I was pretty delighted with Xtro, the central villain of which never seems to take the same form more than once. And I don't mean it changes like the Alien in Alien, or the Thing in The Thing. The various threats to the characters manifest in such different, crazy ways that the movie often comes off more like particularly imaginative child's nightmare than a conventional horror movie.
I'm going to try not to say too much about the plot, but it really doesn't matter anyway - if I DID describe the plot at length you would probably assume that Xtro fell into one of those predetermined groups (body-snatcher movie, to be specific). But instead of just turning into emotionless drones, the aliens' victims...well, suffice it to say it's quite a bit more complicated (and fun) than that.
I'll give just one example of the creativity overflowing from Xtro, which by the way was intended as a sort of British knock-off of Alien: Towards the beginning of the movie, an unlucky couple runs into one of the movie's monsters - a creature with such a startling and memorable design I figured he'd be seen a lot over the course of the movie. Nope. We never see him again! And just when you think you have Xtro figured out - there's a scene with a giant GI Joe action figure come to life attacking an old woman with a bayonet. God Bless Halloween.