George Lucas was amazing in many ways. He and Spielberg achieved the dream that their film group (American Zoetrope - founded by Francis Ford Coppola) set out to achieve, which was to make major movies outside the direct influence of the Hollywood system.
George changed how the industry looked at special effects, movie sound, visual quality, integration of modern and just-developed equipment, distribution, merchandising, and other areas. He also knew how to hire great people and hold high standards.
What George is NOT good at is WRITING or DIRECTING. His original script for Star Wars was pretty horrible, and the only reason it is as good as it is was because he had Francis Ford Coppola re-write it. George's writing is lousy, his dialog is horrible, and his direction is as wooden as it gets. The smartest thing he ever did in this regard was higher quality directors for Empire and Jedi and stick to the parts he did well.
Lucas admittedly stole storylines and shooting sequences from other movies - as gray_sailor says, Star Wars is his take on a modern version of the serials he grew up watching as a kid. He used language and concepts from Flash Gordon for the "space stuff", and borrowed heavily from famed Japanese director Akira Kurosawa (as many have), in particular from The Hidden Fortress and Yojimbo (some bits of these movies are almost directly translated into Star Wars). For the space battles, much of it is shot-for-shot remakes of a WWII movie called The Dam Busters.
Lucas also incorporated the ideas in Joseph Campbell's book A Hero With A Thousand Faces, which breaks down how most "hero" stories are all built around the same progression of concepts, and you can see that Star Wars follows this exactly.
So, yes, Star Wars, as a raw story, is very unoriginal. The acting is often lousy (more due to bad direction than bad actors). Absolutely true.
But no one your age can understand what a breakthrough Star Wars was in so many other areas.
Visually, it was so much more advanced than anything before that its like comparing a rotary telephone to the latest iPhone. The same for the sound - most SciFi movies before Star Wars had a mono soundtrack, while Star Wars had Dolby Surround.
I could go on, but the point is that you grew up in a world where Star Wars was the standard. Some of us were around to see Star Wars BECOME the standard.