Early reviews of the first few movies state that they are the equivalent of Saturday matinee Republic serials, and they likely are as Lucas wanted to remake Flash Gordon. This is also seen in allusions to movies like the one just mentioned, various westerns, pirate flicks, and even war movies like Dam Busters:
However, he also borrowed from what he saw in film school, like Kurosawa's works. That plus what he read from Campbell allowed him to make his own stories, but what made them immersive and sophisticated are the special effects, which is what I remember impressed not only kids but even older audiences.
In time, viewers got used to them, so he tried to make stories more sophisticated by bringing in political intrigue in the prequels, but it didn't work.
Finally, we have the sequels, which essentially borrow from the earlier films and cram in more spectacle.
I admit , id like to have seen him do Flash Gordon .