In The Movies: The Force is the mystical power that binds all living things together, good or evil. It's the universal connection between people that gives Star Wars much of its philosophical resonance. In the words of Yoda, as he pinches Luke's shirt, we are all "luminous beings, not this crude matter." (It's also possible this was just a comment on Luke's wardrobe choice. I know it's the swamp, but ribbed tank tops? Seriously?)
In the Expanded Universe: The Force is the power that ties all living things together
unless they're, like, REALLY scary and evil. Like the Yuuzahn Vong. Introduced in the New Jedi Order series, they're a race of self-mutilating bad guys from "outside the Force," "outside the galaxy," and, for all intents and purposes, "outside Star Wars." Making the Force essentially a localized phenomenon does more to neuter it as a concept than any midi-chlorians ever could. The whole story arc reads less like Star Wars and more like some poor soul's Hellraiser-crossover fan fiction.
In the Movies: Luke confronts the Emperor, denies the Dark Side, wins over Vader, and saves the galaxy. The line, "I am a Jedi, like my father before me," cements Luke as his own man free of his father's destiny.
In the Expanded Universe: Never mind. Luke runs into the Emperor's clone on Byss, has a brief conversation, and agrees to be his apprentice. The Emperor ultimately convinces him by offering leadership of the Imperial fleet. The deep emotional connection he felt with Vader and the prospect of ruling the universe as father and son? Not enough to convince him. But let that dude boss around a few TIE pilots and he's totally game for some Dark Side action!
Yes, Luke is QUICKLY recovered by his friends, but having Luke succumb to the Dark Side is against everything the character stands for. Star Wars is the story of Luke not making the same mistakes as his father. Now he's just a shittier Darth Vader. This literally undoes Star Wars.*
*Hyperbole, but still, IT WAS A PRETTY SHITTY THING TO DO.
In The Movies: Obi Wan cuts Darth Maul in half. It's a great crowning moment to the movie's best action set piece, and also shows that Obi Wan isn't a perfect warrior-monk, but a real human capable of anger and vengeance. Also
he cuts a dude in half with a lightsaber. It's pretty sweet.
In The Expanded Universe: The prequels introduced us to many, many, awful things, but at least they never brought back a clearly dead villain, gave him robot legs, and had him hunt a baby Luke Skywalker. We'll give you a few seconds to re-read that sentence. That's right in the EU comics, the top half of Darth Maul somehow climbs out of the reactor pit, finds robot replacements for his legs and major internal organs, finds his way to Tattooine, and threatens three-year-old Luke as a way to draw out Obi-Wan Kenobi. And that was the last time Dark Horse let a fourth grader write a comic book.