Maybe having grown up with the movies I am a little biased, but I have always found the plot of the OT to be very tangible, the acting to be decent enough, and the dialogue has never seemed lame to me.
As far as films go, I would say Star Wars and Empire Strikes Back are about as near perfect as you get. The reason why they are so popular and elements of them have become popular culture, is because so many people went to see them. Why? Because they were great films. They were not summer blockbusters that were forgotton a short time later. Do you think we will be quoting lines such as "This is Sparta!" 30 years from now? Not likely. But I have no doubt people will still be quoting the OT 30 years from now.
At anyrate, the OT was very well written. People who claim the PT is just as well written as the OT have no idea what they are talking about.