The Rebels got lucky. Han, Luke, Leia, that Nib-Nub guy who flew with Lando, all of them should have been death-starred hard in the face by the Empire -- and not just because of the Empire's superior numbers and technology.
Darth Vader and his underlings planned every last subtle detail, right down to the color of the spaceships and Vader's own robot voice, according to what science says works.
At the beginning of Episode IV, stormtroopers blast the crap out of some rebel scum. As the smoke clears, out steps a 6-foot-6-inch black-suited man-giant. We realize immediately that this dude is bad as hell, and he's done nothing more than appear on screen for a few seconds.
Also, he does this.
And It Works Because ...
We know what you're thinking: Big deal. Bad guys always wear black. It's an easy way for the audience to know they're evil.
Yeah, but ... why? Why would that one color have any emotion connected to it at all, let alone fear and intimidation?
After all, black also makes us more aggressive. Researchers back in the late 80s noticed that hockey teams in black uniforms earned more penalties and generally played a meaner game, and not all by mere coincidence. Teams that switched to black mid season were suddenly visiting the penalty box and calling the ref a dick a lot more often, too.
The answer may go back to our built-in fear of disease and uncleanliness; experiments show that people instinctively connect black not only with evil and immorality, but contamination and sickness (if you noticed a patch of skin or one of your teeth turn black, you wouldn't assume it was a good thing).
By the way, there's no way he could've brushed his teeth with that thing on.
So when people see black, to this day it serves as an instinctual cue to thoughts of death and evil that, at least in Western cultures, motivate feelings of aggression.
And choketastic space magic.
In the Star Wars universe, just like ours, it means that people will be more likely to crap their space pants when they see a Sith Lord decked out in black.
Instead of this.
Darth Vader talks a lot about crushing the Rebels, bringing his son into the family business and ruling the galaxy, but you'll also notice that he's always either gripping stuff (e.g. his belt, lightsaber, chair arms, rebel throats) or just clenching his fists, like when he walks into the Hoth Rebel base or is trying to convince somebody of the power of the Dark side.
"They have amazing health coverage."
In fact, the only time Vader's hands are completely flat, in any of the movies, is when he's blocking the shit out of lasers.
And It Works Because ...
Experiments show that physically gripping something boosts your endurance and mental willpower. This has been proven in a whole series of experiments; in one, they found that people could endure pain twice as long (in this case, from holding their hands in icy water) if they gripped an object tightly at the same time. Yet another study recorded that students were better at resisting the temptation of junk food at the campus store if they went in all fisty and tense. Making a fist simply makes you stronger.
And fisting someone makes you both stronger.
And you know what makes you stronger still? Thinking evil thoughts. Yes, they did a study at Harvard by simply having three groups hold a 5-pound weight for as long as they could. They'd try it once with nothing at all on their mind, try it again when envisioning themselves doing good deeds (writing a story about themselves where they do nice things) and try yet again when thinking "evil" thoughts (i.e., being forced to write a story about themselves in which they turn to the Dark Side).
The worse performance came when they were thinking about nothing in particular. Thinking good thoughts added about 10 seconds to their endurance. But the Dark Siders beat them both. Kind of explains why Luke could overpower Vader when he got super pissed, gave in to the Dark Side and then, like two seconds later, totally said he hadn't.
Vader is not a humble man. The first time he gets face-to-face with Luke he tells him, "Join me, and together, we can rule the galaxy as father and son!" When they meet again in the next film, with equal confidence Vader says, "You underestimate the power of the Dark Side. If you will not fight, then you will meet your destiny."
Hell, just look at the way the guy stands. He fills the room.
This is how to win friends and influence people.
Luke tells Vader and the Emperor that overconfidence is their weakness. He's totally wrong.
And It Works Because ...
First of all, just standing like Darth Vader raises your testosterone levels, according to studies. It's evolution -- acting like the "alpha" in the group with your body language actually creates changes to let you assume the role -- physically.
Sort of like how dressing in a cop uniform is enough to get free drugs out of people.
And while we associate pride and arrogance with lazy douche bags who are just bound to get their comeuppance, a study at Northeastern University found that people who had been given an ego boost before setting out to complete a task worked longer and harder than their more humble peers. Their arrogance made them more effective.
And not only that, but they were also viewed as more dominant and likable by other participants. Experiments show that as much as we say we hate arrogant or prideful people, when we run across people acting like hot shit, we tend to assume they are.
Hell, "acting like hot shit" is the whole reason for Boba Fett's fame.
So Vader's boastful arrogance also would be instrumental in convincing Star Destroyers full of underlings to do what the fuck he says.