So I recently saw a special re-release (as part of a "Classic Movies" event wherein the cinema has old movies every week with limited showings) of Back to the Future. It was fantastic: I saw details that I never noticed before and probably aren't visible with standard screen sizes (in case it isn't obvious, I don't have a huge home theater setup). It was then I decided that they really shouldn't have made any sequels, as sometime after the release of BttF someone decided it should be a trilogy. While II and III weren't bad movies (though the late Roger Ebert wasn't too fond of them), it created all sorts of problems for continuity and introduced numerous new plot threads. Here's why they should've stayed at one movie. In case it isn't obvious, major spoilers ahead:
1) They replaced Claudia Wells (Jennifer) with another actress, following some sort of breakdown Wells had. They also replaced Crispin Glover (George) after he requested too high a fee. Actor changes are never good: that was one of the reasons I didn't like Iron Man 2 too much, because they replaced Rhodes' character with someone else who looked and sounded nothing like him (and frankly, after seeing Hotel Rwanda, it didn't seem like a good fit for the new actor anyway)
2) Showing 2015 was ridiculous, as it was fun to see hints of "the future" (specifically, the flying car and Mr. Fusion) but not so much to actually have Hill Valley 2015. Silly, stupid, and unnecessary.
3) Even after being punched out by George in 1955, Biff is more than ready to repeatedly kick Marty in the stomach, and in 2015 Biff is still a bit of a bully. This doesn't introduce, but makes worse a problem from the original film: if George stood up to him, then why is Biff working auto detailing instead of bullying and building off the success of another schmuck?
4) Creating a character flaw for Marty in between two movies? And in the Original 2015, he suddenly seems to forget everything he learned in the first film after the wreck with the Rolls-Royce in 1985?
5) The multiple playing of actors is weird: the enhanced breasts/wig/gaudy clothes Lorraine of 1985-A, Thomas A. Wilson playing Old Biff of 2015, his mentally unhinged grandson, and Buford "Mad Dog" Tannen. And let's not forget Marty, who plays Old Marty (2015), along with his son Marty Jr. (who looks exactly like 1985 except with greasy, stringy hair) and Marty's daughter as well (the less said, the better)
6) They had to destroy the DeLorean in the end. And the flying train was ridiculous, along with a similar flux capacitor which makes even less than needing plutonium to run ("it runs on steam!")
The original ending used the time-honored "And the Adventure Continues" ending, in which it was vague (the filmmakers didn't know there would be a sequel at that point) but could be used as a sequel hook if necessary, but it also makes Doc a liar as well ("Oh no, both you and Jennifer turn out fine" and "This concerns her, too").