1. Red-haired men would be sex symbols
Redheads have it tough. We are routinely mocked, desired, and openly loathed. But while titan-haired women can often be in fashion, ginger men never are. Everyone is frightened by their pale skin, freckles, light eyes, and hair. They are constantly mocked. They are never considered sexy. But had Eric Stolz played Marty McFly, this would have changed overnight. Stolz would have been an ambassador of gingers. Girls loved Marty and guys wanted to be him. His adventurous pluck would have been seen as a natural accompaniment to his fiery hair. He would have been seen as the second coming of Danny Kaye (not a real redhead, by the way), and every guy would have been dying his hair Electric Copper. Stoltz, David Wenham, Kevin McKidd, Rhys Darby, and Scott Grimes would be A-listers ruling the box office. George Clooney would wonder where it all went wrong.
2. Fluorescent future fashions would have rocked the 1990s and beyond
Forget 2015! We would have enjoyed all the boxy jackets, plastic headwear, and chunky high-tops by 1999 or 2000. Grunge and Chuck Taylors would have lurked in the corners, an angry and earthy alternative to the lurid phosphorescent. Those who wore it would be as ruthlessly mocked as hipsters are today. By 2010, we would have streamlined it into something that resembled Star Trek: The Next Generation or Mass Effect wear. There would be no skinny jeans, no American Apparel, and no creepy Urban Outfitter stores.
Why? Well, it goes back to #1. Few people could have matched Stolz's fiery locks, so they would have compensated by dressing as luridly as possible. Also, redheaded men don't really look good in any color, so they might as well wear unearthly tones, and capture that Space Oddity vibe. It ushers in a future of fashion by way of David Bowie.
3. Tom Cruise would be in Caprica, Eric Stoltz would be in Mission: Impossible 4
How is this possible? Easy. Stoltz was a contender for the role of Pete "Maverick" Mitchell in Top Gun. Michael J. Fox was too, but he turned down the role. If Stoltz had been in BTTF, he would have had all kinds of sexy buzz, and he would have been offered the role of Maverick over Cruise. He would have ruled the rest of the 1980s and 1990s as a result. A Few Good Men, The Firm, Jerry Maguire would have all starred Stoltz. Far and Away would have been really heart-wrenching and believable, garnering 10 Oscar nominations. He may have still dated Katie Holmes, but he's more reserved, and probably wouldn't have done the great couch jumping stunt or Tropic Thunder. Imagine a world without Tom Cruise (he's around and he's always been popular, just never a huge star) or Scientology as a reference. Already, it's kind of frightening, isn't it?
4. There would be no Doc Hollywood, so there would be no Pixar's Cars
Without Back to the Future, Michael J. Fox would probably have been relegated to television roles. (Much like Stoltz.) So he would never have gotten a role like Doc Hollywood and the other stars of this hypothetical 1991 are simply uninterested in such a distinctly unflashy film. So it doesn't exist. Because it's never made, Cars is never made because no one can think up a sensible plot involving a race car. Without their merchandising cash cow, Pixar can't make Wall E or Up. The entire world of animation ceases to be exciting and engrossing, and millions of little boys wake up with a sense of yearning and loss that can't be explained.
5. Back to the Future Part III would be set in Ancient Rome or Tudor England
BTTF 3 was set in the West because Michael J. Fox wanted Marty to go back in time and meet cowboys. With Stoltz in the lead, 3 would have taken place in Rome (Christopher Lloyd's dream destination) or Tudor England due to Stoltz's interest in the theater. There would have been hijinks with a Senator Biffus Tannitus, opponent of Julius Caesar, or Francis Tannen, a competitor of William Shakespeare. The ripple effect would have been small, but depressing. Without the gratuitous references to Clint Eastwood and A Fistful of Dollars, an entire generation and a half skips Sergio Leone, and Westerns go completely extinct. They aren't even referenced in Wolverine comics because no one will understand the joke. It goes so far in this 2010 that the Coen brothers anticipated remake is their version of Titus Andronicus. Weird.
6. There are hoverboards!
It's true! Here's how it happens. The character of Marty McFly was an avid skateboarder, and Tony Hawk was originally the skating double for Stoltz. He lost the job (though he stayed on to coordinate the stunts) when Stoltz did, because the height difference between him and Fox was too obvious. But because Stoltz and Hawk remain employed through BTTF2, they are seduced and obsessed by the possibility of flying skateboards. They continue to invest time and money in research technology, and campaign in Washington for scientific funding until hoverboards are a reality.
Now, don't take this as a mean criticism of Fox. He liked skateboarding, but he was already married and a father by 1989. He had grown out of a need for a hoverboard, and had more important things to do and campaign for. Stoltz was (and remains) a bachelor. He now has millions of dollars because of his A-list career, and a daredevil streak necessary to keep pushing for them. This is how we end up with hoverboards.
So, there you have it. If Eric Stoltz had played Marty McFly, you'd be dressed in Star Trek clothes, watching comedies starring Rhys Darby, and admiring Tom Cruise on Caprica. You would have no Pixar, but you'd have hoverboards.
But chances are good you probably wouldn't even be all that excited for the 25th Anniversary of Back to the Future because they're kind of dated and a little mopey. Stoltz was really cute as Marty, but he took the character so seriously. They would probably have been better movies with someone funnier in the role -- maybe that Michael J. Fox guy, because he's so cute on How I Met Your Mother. Plus, the series really went downhill with the third one. It would have been so much more believable if the third one was about the Wild West. Why would Biff have an ancestor in Ancient Rome, anyway? That was just dumb.
Oh, who talks about movies on the Internet anyway. That's for real nerds and losers. Everyone else has hoverboards to ride.