I've concluded that if a bike fits well, runs quietly, and basically performs acceptably then it can be a joy to ride, cheap or not.
As I keep riding, good bike FIT gains more importance to me. When a bike has some unexplained, constant creak, ping, or other noise, it really bothers me.
"performs acceptably" is open to a lot of interpretations. I'm not a sophisticated, detailed cyclist. For me "acceptably" its not much more than having brakes that stop the bike, being able to change gears without too much "clunkiness", and having a frame and wheels that are not obviously out of true.
More money can buy a lighter bike with (I assume) smoother/better gear-changing and braking (than your 70's era steel bike), but I doubt it would be any more durable. As far as bike weight is concerned, I used to be into wanting a sub-20 pound bike, but after considering the way I ride (I'm not a racer), and all the information considering bike-weight that I've read here on bikeforums.net, I don't think it would add that much to my enjoyment of biking (which way the wind is blowing on a given day would probably have more impact on my ride).
I do like aliminum-alloy rims (which will bring a lot of steel-rimmed bikes to sub-30-pound weight)