Re: "Joyless" ?
It's funny to notice so many similar stuff as when fans were analyzing/criticizing Superman Returns. Being too dark and depressed was also one of the main criticisms of SR, as was making too many references to Donner movies, which catches my attention since MoS took quite some things from those movies as well (like the overall Krypton-too-Earth Zod's revenge story arc or the S as a Kryptonian symbol).
What killed the movie for me were two things: Firstly, Pa Kent's death. Well, maybe not his death but whatever the writers and directors wanted to say with that. To me it was more shocking than the sense it made. And Clark letting him die was even more unreasonable. Apparently, Pa Kent wanted to stop Clark from saving people. He voluntarily died and Clark went on to... yes, save people. It was never clear what was that about, why Clark didn't do a thing and why, apparently, that death had no effect in Clark's decision.
The other thing is, as some people as hopefuldreamer have mentioned, Superman's heroism. Or at least what the people's perception of him was. Why he is called "Superman." All what people of the earth saw - at least 99.9% of them - was an alien invasion. That's it. And one of the Kryptonians was good and saved people. But never in the movie the world sees this man that's superior to any other man. Because in fact they saw a whole bunch of them. Superman didn't get the chance to be a super-man because he was just one of all the super-men that invaded earth.
I don't mind Superman killing. But it would have been great if his aversion to killing was well established before him killing Zod. Naturally, that way the scene would have gathered more importance.