Like every other movie shot pre-1990's, Back to the Future was shot on film. Which should make most indie filmmakers happy. And probably makes James Cameron a tad irritable.
A lot of us are pretty jaded nowadays. Every sci-fi film is a roller coaster of lights, movement and sound. It's pretty rare to see a movie in the genre that slows things down just a bit, and looks like it's actually happening, rather than just a slew of insane—albeit technically impressive—special effects. Okay, so once in a while it's pretty obvious there's a green screen, and we don't really ever think that Marty's hand is going bye-bye, but it still feels a bit more homemade. And not in a bad way.
It definitely helped when it came to crafting locations that we would fall in love with and that we would feel like we could just step right into. Much of the movie may have been shot on a set (including all the Town Square stuff, which was filmed on the Universal Studios back lot), but we will never look at that Clock Tower and think of any other film, or see any storefronts positioned around a town square and not associate them immediately with BTTF.
The filmmakers wanted us to feel like we were in a cozy, familiar setting, and they accomplished that by building a world that didn't seem too far off from our own… minus the curious surplus of manure trucks, of course.