As Manofsteel1979 said, that scene from Clark to Superman and back again made me see how he could fool people with his disguise. Christopher Reeve once said that the approached it as he was playing two different characters, which really sells it. I thought that was a brilliant move.
This movie is definitely of a certain time but I love it all the same. One subtle thing that I always loved is that when Superman flies through Metropolis, people stop to look up in wonder. I just love that. It's such a little thing that says so much.