I think good acting, original ideas, and good deaths/scares. Any movie can have jump-scares, but it takes a great one to make you have a very creeped out feeling all the way through. Saw really comes to mind. (forget about the sequels) Saw as a stand alone movie was actually very creative. The acting wasn't terrible, and the deaths were realistic.
So any horror movie that actually does something new, is fine by me.