Good versus evil is easier when neither side are bumbling fools. Darth Vader is one of the great film villains despite the simplicity of his motivation--hell, in the original Star Wars, does he even have a motivation? He's evil, he's prone to choking people, he's not quite in charge but his powers mean most other imperials defer to him. But, why is he this Imperial... Tool? Weapon? Knight? His look is iconic enough--and James Earl Jones' voiceover performance as well--that he works in a film aimed at a young audience.
George Lucas has a good eye for set pieces, especially when he's not free to do whatever he wants with CGI. He is good at throwing one problem after another at his heroes like we're watching some old serial in some small town theater in the 40s. And here, with The Empire Strikes Back, he's just far enough away from the production--as neither director nor screenwriter--that everything works a little better. I mean, it's not just the better grip on romance, the more direct, and better choreographed, battle between good and evil, or the bleak ending that makes this one most people's favorite Star Wars film. It's also got direction on its side. It's got better acting from its leads; since this is three years on (and Hamill has been in two more films, Ford had been in six more, Fisher had been in three more, and they all had made that amazing Star Wars Holiday Special) it would really suck if they couldn't manage it. Even Chewbacca's side eye is on point.
For example, introducing Vader's newer, bigger and badder Star Destroyer by casting its shadow over one of the smaller ones is brilliant filmmaking.
Admiral Ozzel (Michael Sheard) has got nothing of Grand Moff Tarkin's charisma. Poor guy. Can't hold his own against David Prowse in a plastic helmet. It should have been no surprise at the time that he wasn't going to last long. Now, he's just another guy Vader kills (even though he really doesn't kill that many, it's all just so familiar that he's basically a mass murderer... To be fair, in the first film, Obi Wan did say that Vader "helped the Empire hunt down and destroy the Jedi Knights" so he's got that going for him.) when his death should be a shock. But, in a dictatorial system like the Empire, especially on its military ships, I guess promotion by murder is just another day in the Imperial Navy. The guy under Piett (Kenneth Colley) is just waiting, and hoping and dreading, for that moment Piett screws up.
Another bit of brilliant filmmaking--though I would never have noticed it as a kid--is that showing us Vader without his helmet (even if just from behind) early in the second act is clear foreshadowing about his identity being important, and likely being revealed, later. As a kid, though, I just thought it was cool, because he's this scarred bald guy wearing an awesome suit.
Even if he drops everything when the Emperor beckons, Vader is still a badass.
Returning to an old standby is always nice. But, even better is when I notice something new. I've always been amused by this exchange--
Han: No time to discuss this as a committee.
Leia: I am not a committee!
--that I failed to notice that as Threepio loses his balance in the background he screams. I've known this film for 37 years, and it's still got surprises for me.
Luke's facing "Vader" in the cave on Dagobah and finding his own face under the mask--that's the obvious foreshadowing.
(Weird Expanded Universe note: the Dark Jedi that died in that cave, fueling it's dark side energy, apparently never got a name. I thought everything but Yoda's species got a name. He was killed by a Jedi, of that same unnamed species as Yoda, called Minch. Of course, this story is no longer canon.)
You know what Yoda needs? He needs to be far more clear. He tells Luke to remember his "failure in the cave" but all Luke did in there was face Vader, not run away, and win. Sure there's the creepy hey look, it's my own face moment, but how is that failure? Yoda needs to be less cryptic.
I've seen people point out how badass Han is for immediately pulling his blaster on sight of Vader. But, Luke did it first, striding forward and getting into a light saber battle he has had barely any training for. Hell, Luke is more of a badass, because Han has clearly been using a blaster for a long time; he knows what he's doing.