You said it. Just like in real life, things have no "endings," they just flow forward somehow. There's no magical deus ex machina or beautiful speech that make everything going right at the last second. SR ala STM or SII would have Superman reversing time or deleting memories so Jason was never born or Lois won't be able to remember Superman left her without saying good-bye.
You were responding to someone else when you posted this, but I wanted to pick up on a few points.
It used to be the tradition in fantasy stories that there was a happy ending, or a neat ending, or even that things returned to the status quo - the way they were. I think the amnesia kiss and reversing time in the earlier Superman movies of the 70s/80s are part of that previous system in fantasy stories. In the classic movie version of Wizard of Oz, it's all a dream for Dorothy; and an entire season of Dallas was a dream too. Those sorts of plot devices wouldn't hold up today and I don't think anyone watching SR wanted anything like that to happen.
It's not the events at the end of the movie that are the problem (I'm not calling for amnesia kisses or time reversals); it's the events BEFORE the movie that are problematic. The relationship, the pregnancy, the leaving of Earth, the not saying goodbye.
I saw the SR hardback annual (children's book) in a second-hand store on Friday and the telling of the story in there made more sense. Superman had been convinced that there was still life on Krypton's remains, the dialogue and explanation were much clearer.
That is another quality I didn't mention. Conflicts are clear enough so they don't have to be spoonfeeding to us verbally. Sadly that happens too much nowadays. For me, movies that had everything to be great (as Batman begins for example) are toptally ruined by over-explainative repetitive dialogues.
Some clarity is needed; we cannot base stories on vague history. Otherwise it's like being dropped from a helicopter into an unknown landscape with no map, no idea of where we are and what direction we are going in.
What defined something else for me was the way Superman, while not lying directly, hides the truth about him and his actions all the time. And saying “Oh I went for a juice” when Lois asks Clark where was he while Superman rescued that kid. I understand Superman’s mission requires he to distort truth in order to protect his identioty and mission. But then don’t go all “I never lie.” I could, for example, be okay with the amnesia kiss if he offered to forget everything to Lois before doing it.
I've always thought that Superman doesn't lie. It's Clark who lies, and acts like a nerd, to protect Superman's identity. There is a great episode of the TV show Lois and Clark (it was on TV here just the other week) where a villain has apparent evidence that reveals Clark is Superman - and Clark thinks he will have to admit it to everyone, but Lois says that there is a 'greater truth at stake' and that the world needs a mythic hero. At the press conference, Clark decides NOT to tell the truth and covers it up with a story. Although Superman stands for protecting the truth (=honesty, goodness, belief, faith in something better) of the nation/world, he must hide his own truth in order to do that. It's important that he appear as more of a symbol (of hope/strength and doing the right thing, the good thing) than as a person with vulnerabilities that can be exploited. Notice the world is never shown questioning who he is and what he does in his daily life when he isn't flying around, they happily accept this symbol of goodness.
In a way it's like God, like religion, though Superman is NOT God. If people believe in God, they don't constantly question the truth and identity and physical reality of that God, they don't ask who he/she/it is. People have a need to believe in something greater than themselves. In today's crazy world, that might mean believing in aliens and spaceships or it might mean deifying things that are not deities (celebrities usually). Superman represents that 'something greater than ourselves', a source of inspiration, who stands for that 'greater truth.'