Presence: the man had presence.
If you grew up at any time from the early 1950s right up through 1978, there was only one Superman. George Reeves had, somewhat reluctantly, made that role his own for more than one generation of TV watchers.
Plagued by budgetary constraints and nurtured by scripts that ranged from genuinely compelling to absurdly awful, The Adventures of Superman had one steadfast, invincible constant: its star. As played by Reeves, Superman was no angst driven, soul searching hero. He was the "go to guy," the man with the answers. This Man of Steel's moral certainty and beneficence were as unassailable as his invulnerable frame.
Reeves was completely believable as both all purpose problem solver and unbeatable hero. This was perhaps due, in part, to the fact that he played Clark Kent in pretty much the same manner that he played Superman. Contrary to statements made by another of this page's reviewers, this was not the way Kent was portrayed in the comics of the period. Reeves performance, however, almost certainly inspired that particular evolution of the character, when John Byrne took over the comic book in 1986.
In addition, Reeves, with the occasional aid of the scriptwriters, injected some real humor into the character. It's a great responsibility to be Superman, but it's also a lot of fun. Reeves allowed the audience to be in on the joke with his smiles, winks and, once in a while, an intentional verbal slip. In one episode, when Kent and Superman are mentioned in the same breath, Clark half mumbles, "Six of one. Half a dozen of the other."
Remember the scene in Seinfeld, when Jerry and George argue over whether or not Superman has super humor? Jerry would have to look no further than this series to prove his point. Humanity is very funny to this Kal-el. He doesn't change his hair, voice or manner. A blue suit and a pair of glasses and suddenly your best friends don't know you.
Oh yes, there is the matter of the invulnerable hero who ducks when a gun is thrown at him. Sometimes it played out that way. Sometimes he'd let the gun hit him, and other times he'd grab the gun and crush it. The truth is that those fake guns hurt. But how can we be sure? That's evidence those crooks are throwing around: evidence that would suffer less damage striking a concrete wall than slamming against a stronger than steel chest. If you want to take Superman to task, complain about him crushing evidence. But then, as I say, he was a Superman who liked to have fun.
Even after the series switched to color, Superman lived in a black and white world. There was right, and there was wrong. It was simplistic and unrealistic. Still, even as kids we knew that there would be times in our lives when we would face situations where the choice was pretty clear. We would see the right, and we would see the wrong. At times like these, thanks to George Reeves and Superman, we would know exactly what we had to do.