1) Stormtroopers are Precise?-In A New Hope, upon finding the sandcrawler that sold him R2-D2 and C-3PO destroyed, Luke Skywalker assumed it was done by the sand people (AKA: Tusken Raiders-are they Italian?). However Obi-Wan Kenobi points out that this is not the case due to the Bantha tracks in the sand. He then moves Luke closer and shows him the blast marks.
Obi-Wan: "And these blast points-too accurate for sand people. Only Imperial Stormtroopers are so precise."
Remember that last sentence because the next time we see Stormtroopers trying to shoot something, their target is the unsuspecting (and momentarily stationary) Han Solo just as he is about do leave Mos Eisley. There are about five or six Stormtroopers shooting and not one of them come close to hitting Han. In fact, I can think of only a few instances where a stormtrooper has managed to hit his target and trust me, there aren't a lot.
2) Who Needs Sight to Land?-In The Empire Strikes Back, Luke goes to the planet Dagobah to train with Yoda. He moves to bring his ship down but is quickly blinded by all the fog. R2-D2 is beeping like crazy because he thinks he's riding shotgun with Billy Joel and Luke tries to calm him down.
Luke: "All the scopes are down; I can't see a thing! Just hang on, I'm going to start the landing cycle."
I know Luke is supposed to be a good pilot but to land a ship without seeing? Holy shit, the terrorists in Die Hard 2 would have been royally screwed if Luke was flying one of those planes. Maybe the Force was with him but given how he lands, I'd say it was dumb luck. Or destiny. That word is always crawling it's way into Star Wars scripts.
3) How did Vader Know Luke Would Come?-OK, we know in Empire that Han, Leia, Chewie and 3PO are captured by the Empire at Cloud City to lure Luke Skywalker into a trap. There's just one problem: How do they know that he'll come?
I'm not kidding, it's never explained how Vader knows that Luke is aware his friends are in danger. I mean, for all Vader knows, Luke is partying with the rebels after they escaped from Hoth. He doesn't know Luke is training with Yoda. I mean, did they make an announcement over intergalactic communications?
And suppose Luke didn't go to Dagobah?
What if Luke said, "Ah, screw going to Dagobah. I'm gonna go to Sullest where the beaches are fine and the bitches are finer!"
That means he wouldn't have developed his Force abilities, wouldn't have sensed his friends being tortured and wouldn't have gone to Cloud City. Vader would have been like that kid who's still waiting for his deadbeat dad to return from the corner store even though he left five years ago.
4) Obi-Wan's Interference-Since Luke did go to Dagobah, he used the Force and has to leave to save his friends despite Yoda insisting that he stay and complete his training. However Luke feels he must go because Han, Leia and Chewie are his BFFs.
All of a sudden Obi-Wan appears all sparkly and ghost-like to appeal to Luke. However he and Yoda cannot get through to young Skywalker. They know he's not ready to face Vader but Luke feels he must if it can save his friends. Desperate, Obi-Wan tries just one more time.
Obi-Wan: "If you choose to face Vader you will do it alone. I cannot interfere."
Suppose Obi-Wan could interfere...what the hell would he do? I mean, he's dead! I know he can appear all sparkly and glowy but he's a ghost. He's not going to do much good if he can't move anything on this plane of reality. The worst he could do is try to scare the shit out of Vader but I doubt that would work. That or as Luke and Vader are fighting he could just keep saying, "Hey, cut it out! Stop that, I mean it!!! Hey stop it, someone's going to get hurt!!! I mean it, I'm going to get very upset soon."