But other sequels have cliffhanger endings that people love as sequels, such as The Empire Strikes Back, or The Dark Knight, so does it really put people off for Part II?
I think the cliffhanger on Empire made people want to watch the next one.
The cliffhanger on BTTF made people go "Fwoooar!!" and made them yearn for more, simply because the original film was so much fun overall.
The cliffhanger itself was well done, because it was: Talking about the future, the car flies, the car then flies at the camera, a flash of light, cool music... there was promise of adventure in the cliffhanger.
But the BTTF2 cliffhanger was almost a bum note in the way it played out, especially at the end of such a fun movie.
Huge fun during the film... then Doc simply passing out when Marty reappears and the camera panning out? It's kinda boring.
Then they had that end-credit sequence showing what was coming up in part 3.
I think it kinda spoils the anticipation, and leaves part 2 as a loose end.
I still prefer part 2 over 3 any day though.