Whoever said the tone of a Superman story has to be uplifting and happy sunshine and rainbows doesn't understand the character at all.
I didn't say the tone of a Superman story needs to be sunshine and rainbows. I said Superman needs to be sunshine and rainbows. He's the definitive superHERO. He can do anything. He's the best of all of us. Etc. Again, I'll direct you to the Superman celebration All-Star Superman.
I point anyone who thinks that to the Superman TAS episodes: Fun and Games, Brave New Metropolis, The Hand of Fate, Apokolips…Now!, Unity, Legacy, and ESPECIALLY The Late Mr. Kent. The part where the guy figures out who Superman is at the very end as they show the switch being thrown to execute him was just... wow...
Those are episodes in a TV show where Superman is already well developed and you can allow him to get into darker situations and take darker turns. This is an introduction movie. It needs to show Superman at hist best before you start taking swerves.
Batman represents Darkness, he's the brooder, he's the moody one. He can have pathos and angst. Superman represents the light and is supposed to ultimately be that bright light to usher humanity into good. The story around him, however, doesn't have to follow that same tone. It can be dark, it can go to dark places. Even better that they're starting at the beginning of his superhero career. It gives them the option of having him be uncertain and lost about his position or his way in the world, as long as he ultimately finds it and ends up in his rightful place in the light at the end. That was kind of the entire premise of Smallville, and it came out great.
Smallville was a bad TV show about a whiny teenager. It sincerely hope they're not taking cues from that load. And just because he's uncertain about his powers and his place in the world doesn't mean he can't be rainbows and sunshine.
That's why I like the poster, I think it's very fitting, because if I'm interpreting it right, they are going for exactly what I just said. Look at the poster again, specifically the lighting. You have those soldiers around him all darkened and featureless. Superman himself however seems to be emitting a light or has a light directly behind him lighting him better. He's the bright light in the darkness here. That's the tone Superman should have.
He's shackled, his face oozes sadness, most of his body is still shrouded in shadows, his costume uses very dark tones instead of the more familiar brighter ones, etc.
They probably did go for the "light in the darkness" thing, but it comes off as a fail.