The story is very classic. It doesn't suck, but it's not unique either.
Anyone can follow it, without it being boring. But as you said, it's not the reason that people love the franchise.
Varied characters, memorable script, special effects, great design of costumes, ships, worlds (one of the innovative things was to make it look realistic by making some things ragged, dirty, worn out, instead of all that sleek silver design of the earlier sci-fi'es). And one factor I think people underestimate is the music. I hear the opening theme or imperial march in my head just by thinking of the title.
People crave more comprehensive, deep stories today, and they aren't easy to tell in a two hour sci-fi film. So they tend to favor high-budget television series, where you have time to explore the lives and minds of the characters. Lucas was talking about a TV-show, but I guess that's not gonna happen now...unless Disney makes a "Jedi-Jane & friends" TV-show for children :P
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