Everyone has already covered a lot but my tongue in cheek B-movie input is that the monster has to eat half the cast, they should say "f..." a lot and there has to be naked chicks, gratuitous (or naked guys) that has nothing at all to do with the plot!
Seriously, you need to make it creepy if possible, but also have fun with it if you can. Also, have a decent payoff at the end so it makes it worthwhile for the reader to sit down and invest the time and money into buying and reading it. That should be a given, but I've read some horror and felt ripped off with bad endings. Not everyone will agree but still...it IS entertainment, after all. Scary fun.
Seriously, you need to make it creepy if possible, but also have fun with it if you can. Also, have a decent payoff at the end so it makes it worthwhile for the reader to sit down and invest the time and money into buying and reading it. That should be a given, but I've read some horror and felt ripped off with bad endings. Not everyone will agree but still...it IS entertainment, after all. Scary fun.