Donner's Superman is classic because it was the first of its kind ... not because its execution was perfect. Much like Star Wars. It broke new ground, and, in doing so, it took major risks. It was the first serious, grown-up rendition of the previously entirely-childish super-hero genre. It was the first, therefore it will always be classic/noteworthy ... even when something better comes along (which it has, repeatedly, as the genre has been refined).
The movie itself was very good, but far from perfect. Superman was due for an update ... in a new time, for a more discriminating audience, in a well-refined genre. It was due for better effects, and some course corrections in story, casting and character portrayals.
I thought Man of Steel was largely successful in updating Superman and correcting some of the flaws of Donner's. In its effort to separate itself from Donner, it made some good changes, a few interesting departures from mythology, and a couple of changes I wouldn't have made. I thought it gave more depth to Superman than Donner did ... introducing a more compelling and relatable character, as opposed to a beacon-of-perfection from day 1. I thought Lois Lane was much better portrayed, and Zod's motivation made far more sense. I thought Crowe's Jor-El was 10x more interesting than Brando's, and Snyder's Krypton was a lot more effective. I thought the "first live birth in centuries" storyline was very interesting, and a welcomed departure from previous mythology.
I also thought the idea of introducing Superman before Clark Kent was intriguing ... and the idea of letting Lois in-on-it from the outset was good (as opposed to her being an idiot for a movie or two). On the other hand, I thought some of the character choices for Jonathan Kent showed a complete misunderstanding of the character. I didn't care much for Jor-El's death. And, I'm not too sure about the neck-snap. I also wish they'd have worked-in the "Kneel" line ... and Williams' Superman theme (at least in the closing credits).
Man of Steel wasn't perfect either. But, I thought it was a successful re-imagining for a more refined audience. It fixed some of Donner's mistakes ... and probably made a couple of its own. As expected.
SnakeDoc