What I love so much about BvS and MOS is that ZS does not romanticize the heroes. I believe that if a Superman type being ever showed up, it would turn the world upside down. I love this approach. People would love him, hate him, worship him etc... As much as I enjoy Reeve's Superman, I just don't see a world being so accepting of someone like that. The uncertainty surrounding him among the people and governments would be extremely high. Even with his heroics, people would still be afraid. What if with those types of powers, he ever decided to not use them for good because of some traumatic event he suffers. Bringing in an older, more experienced Batman fits so perfectly with this. Here is someone who has had personal experiences with good people who have turned bad. Couple this with what Superman is capable of and how Batman perceives his career against crime to be a failure, to have lost his faith in the good that is still out there, I found all this to be so interesting.
I also love that, being raised as a human, Superman displays a variety of human emotions. We see Superman doubt his place in the world. Yes, Superman is supposed to be a beacon of hope but I can appreciate the journey it takes to get there. I feel it's disingenuous to have him never waver, with the reaction he has received from the world and the incidents that have occurred since he has donned the cape. Again, being raised human, I fully expect him to show the full range of emotions over the course of both films. And with this Superman, we see one of the most important traits displayed for the character, endurance. Through all the adversity and mixed reactions he has received, he ultimately endures and continues his fight for humanity. Of course this action is not lost on Batman, it brings him out of his despair and provides hope to him, that there is indeed still good out there.
I can understand some people not taking a liking to Superman and Batman in this film as much as they could or have in the past, because here they are not at their best. It is not until the end when we see both characters rise from the troubles that each of them faced. But until that point, it is at times a dark journey but one that I feel is honest, especially with the world's reaction and the circumstances put on them. It is why I appreciate what was done with this film so much.