I think of the word abrupt as being inherently negative, but I'm pretty sure that's not strictly the intent here. Still, it's worth distinguishing between an ending that's abrupt and one that's simply very economical, or ambiguous. Sometimes an extended denouement just isn't necessary, as comforting as it might be to audiences who have been trained to expect it. Sometimes there's just nothing more that needs to be said and it's best not to overstay your welcome. Hitchcock does it in a bunch of his films and it's something I love. I'd say AMERICAN WEREWOLF and some of those aforementioned 50s monster movies fit in the same category. I mean, what would one more scene really add to AMERICAN WEREWOLF? Nothing that isn't already expressed or at least implied in those last few moments, and drawing it out would dilute the punch to the gut impact of what happens. It leaves you lingering on exactly the right thing.
Carpenter I'd say is something else entirely. Those little reversals he does are part of the denouement, effectively occurring outside of the central action of the story. It's like a scary version of saying "but that's another story..."
