The non-spoilery version of the "mostly" involves fight scenes that went on 33% too long and two or three moments of Clark Would Never Do That and a certain amount of Plot Stupid.
The spoilery version follows.
I loved, lovedlovedlovedlovedloved the following things:
1) Zod had an actual motive, and it makes sense, and he's tragic and nuanced.
2) Women. Doing things. Being people. Making choices. Martha fucking Kent, along with May Reilly Parker, is the great argument for the centrality of foster-mother figures as the moral arbiters of superhero universes. (Get me drunk and started on Clark vs. Peter sometimes, and how they fill similar roles.)
3) Earth is full of chicks and brown people, almost like we live here or something*.
4) The superhuman fights look amazing. Kryptonian type people move faster than the eye can follow. Then they pause. Sometimes you track them by the explosions.
5) Henry Cavill is beautiful, and he perfectly channels Clark's serenity when the story lets him.
I had issues with some other stuff, though.
Yeah, the point of Clark is that he's everyman, only with superpowers. He's invincible, invulnerable, invisible. He comes and goes like the wind. The conflict of Superman arises from his ethical choices.
So why are we shown adolescent Clark making solid ethical choices, but adult Clark fails that for... a sight gag? I admit, it's a really nice sight gag, but that's not Superman.
Also not Superman:
*kissing your girlfriend while millions of Metropolians are burning to death or being crushed in collapsing buildings
*not attempting to move the Kryptonian fistfight to outer space, but instead blowing up gas stations and IHOPs as collateral damage
*only giving a shit about the people about to die in front of you (or Lois or Martha) and not the faceless hundreds of thousands dying as Metropolis and Nameless Indian Sea Island are taken apart by alien tech.
The body count in this movie makes Kid Miracle[Marvel]man look like an amateur, but it's weirdly elided. We see civilians mowed down like chaff... but there's no emotional resonance to it until the very end. I found this to be a major mistake, and I think Clark's eventual choice would have had more resonance as an act of war if that had been thought through and addressed.
Superman cares when people die. Even strangers. This is what makes him Superman, and a moral ideal.
Clark can hear everybody on the planet screaming as they die. It's the essential tragedy--the interesting conflict--of Superman**. He can save a hell of a lot of people. But he cannot save everyone.
Also, Jonathan Kent only dies because Clark is too stupid to live. You let your dad go after the dog? You n00b. (Also, under an overpass is a bad place to be in a tornado. Suction. Sorry. Lie flat in a field.)
*paraphrased from scott_lynch
**Other interesting thing: Superman is a mask Clark Kent wears. Bruce Wayne is a mask the Batman wears. Discuss.