Mainly, I think it was just that he was more relatable. In the Silver Age, Superman writers relied pretty heavily on story formulas - Superman JUST manages to avoid having his secret identity revealed, Lois ALMOST manages to rope him into marriage, Superman 'teaches someone a *shudder* lesson', etc., etc. These were repeated over and over again, to the point where you could predict the *gasp* SHOCKING SURPRISE several pages away from where it actually turned up. He basically became a deus ex machina in his own adventures. In the Golden Age, Superman was much more human - he enjoyed himself, he indulged in a wisecrack every now and then, and Kryptonite was much less of a factor early on, so villains had a tougher time beating him, which, paradoxically enough, made him more vulnerable. You see, it was taken for granted that it was practically impossible to beat him physically, so villains largely tried to outwit or humiliate him instead - hence the rise of such villains as the Prankster and the Toyman, not to mention, of course, Mr. Mxyzptlk. As a result, Superman had to use his noggin - he outsmarted his villains more than he punched them. In short, the comic was much more about him as a character than him as a plot device - the formulas that were used so often in the Silver Age were still there, of course, but they didn't feel worn out yet, and didn't show up as often.