Hitchcock's endings are wonderfully precise and concise. They're designed to throw you back into reality with a violent thud, and because the credits were short enough to put at the start back then, he could really do so. It all has to do with being the Master of Suspense - if it's suspenseful right 'til the end, with an absolute minimum of wrap-up, the tension lingers on your mind longer. Now, as for some specific films...
The Birds - What happens next? Oh, and I was interested in that budding romance!
The violent bird attacks were a manifestation of the mother's subconscious/repressed resentment towards any woman who might distract her son from his overly devoted attention to her. As a possible mate, Tippi is her prime target, but the attic attack leaves her so traumatized that her sexuality is utterly destroyed, leaving her childlike in her helplessness. Only then can the mother embrace her as a surrogate daughter, knowing that her son can't fall in love with a blubbering wreck. Thus the movie ends at the story's natural conclusion.
Psycho - The dreary psychiatrist's monologue. Ugh.
Yes, that was a mistake.
Rebecca - I wanted to know more about that head maid chick and her relationship with the evil bitch.
She was obsessively devoted - what's more to know?
Rope - The conclusion here, OTOH, seemed to be drawn out with no real payoff in the end. Stewart figures it out and is horrified ... but it seems like there should be some sort of intellectual struggle here between teacher and pupil, instead we just get police sirens.
Blame Jimmy Stewart for this one - in the play the movie was adapted from, it was clear that the professor had had flings with both boys, so his summoning of the police not only condemns their crime, but their common bi/homosexuality, also. Stewart didn't want to play teh gay, though, so the ending (which, again, is the exact proper ending) loses a good deal of its thematic force.
Vertigo - Yeah, this was too abrupt. Needed time to sink in, maybe a closing scene. Jimmy Stewart comatose again?
I actually agree with this one -
Vertigo is much more of a drama than a thriller, so the standard Hitchcock thriller ending leaves something to be desired. Foreign prints of the movie actually carried
an ending Hitchcock didn't want, where censors demanded that the audience hears that the villain was arrested, and while the scene isn't perfect (there's a weird and inappropriate humor beat at its start), there is something eerily appropriate in its quiet fade.