That's no moon. It's a space station.
-Obi-Wan

It is the film that started it all. It is no exaggeration that it changed movies forever. I think most people can agree on that, while they might have a problem on agreeing how good the movie is. I don't think anyone can watch it today for the first time and be objectionable. The original movie is too much a part of our culture. People who have never watched Star Wars still know who Darth Vader is. They know what a lightsaber and Jedi are. Hell they've even heard of the Millennium Falcon and that's just a space ship.

What ends up happening is the first film is sometimes accused of being shallow. The main reason for this is probably because there isn't much of a character arc to speak of going on. I say it doesn't matter. While there might not be much of a character arc anywhere you still end up with well defined characters that are part of a story arc. The classic story arc of the hero's journey. Again, some might say that this isn't original. I say that a lot of our truly great movies end up being classic stories told in a different way, and the "hero's journey" has never been told like this before.

The reason it's told so differently is because it's told in a living breathing universe that had never been seen before. The fact that we're introduced to this universe seamlessly during the course of the story is an amazing feat on it's own.

Getting back to the characters, I think it's actually the strongest thing going for the film. While they might not be overly complex characters, they are still strongly defined and stay true throughout the film and above all are just really great characters. I don't know if it's the writing, the actors or their chemistry together but the three leads are always a joy to watch interact together. Maybe it's a combination of all those reasons. That's not even including the great Alec Guinness. The end result is some of the most memorable moments in film history.

I won't waste time talking about the special effects, because all that needs to be said is that they were ground breaking but they aren't why the film is great. They serve the story and action but don't drive it. That is how special effects are supposed to be used.