In ANH, Vader can't even throw a scare into Leia despite being encased in frightening armor and standing a foot and a half taller than her. Leia laughs off his threats and calls him smelly. It's only a creaky old man's threats which truly frighten her, Grandma Tarkin (yeah, I said that) uses mere ideas to make Leia quake in her white boots, while Vader is simply the muscle, making her face forward. Elsewhere in the film, high-ranking officers are talking to Vader as an equal, even mouthing off to him until he finally gets cheesed off and strangles a guy by doing a "I'm crushing your head" finger move (and the power of his mind).
Yet in ESB, everybody's crapping their pants at the thought of Vader left and right. Vader is now the ultimate commander of the moment. But what has changed here? Vader dresses the same, carries the same lightsaber, has no more Force power than he used to, and his only real accomplishment between the two events is barely surviving the destruction of the Death Star, which just ain't that scary. Nobody who saw Vader choke a b**** lived to tell the tale, nor anyone who saw him cut Obi-Wan's robe in half. So is it just the drive within Vader to track down his son coming out as more authoritative? Is it that the destruction of the Death Star wiped out all higher-ranking Imperial officials save the Emperor? What is it that makes Vader a bigger galactic badass to the folks in ESB that was missing in ANH?