Ewoks. May not be that major but you said "nothing".
Ewoks served as a caricature for the standard "Natives use geurilla warfare to defeat the evil invaders" and if they're bad, then so are the Na'vi in Avatar. I have yet to hear a single good explanation for all the hate on the ewoks. People complain about them all the time but when pressured as to why, I never get an answer.
How about Luke Skywalker suddenly transitioning from a reckless boy coming to grips with his powers to a cool, collected Jedi Master in about 5 minutes?
There were three years between ANH and ESB, which is when the majority of that transition took place. He spent several months on Dagobah receiving training from Yoda, then there was roughly a year and a half between ESB and RotJ, during which Luke was sent on a number of missions of various types, as well as spending ample time alone honing his skills, including the construction of his own lightsaber.
Luke was 19 in ANH, and almost four years of intense travel and fighting with the rebel alliance is plenty to help him grow up.