4) Tone
The near-complete lack of humor in BvS contributes to its overall severe tone, but it isn’t the only reason the film is so oppressively dour.
Nolan’s Batman trilogy was dark, at times punishingly so, but it leavened the bleaker stuff with flashes of hope. It featured smaller, human moments, like the prison ship defying The Joker at the end of The Dark Knight or Batman apparently sacrificing himself for the good of Gotham at the close of The Dark Knight Rises.
BvS unfortunately doesn’t seem to have much time for hope. It’s a harsh world in this new DC Cinematic Universe, with Batman ruling with an iron fist and criminals and civilians alike dropping like flies.
Even the character of Superman doesn’t appear to have any faith left in humanity anymore – then, by the end, he’s been killed just because Snyder wanted to insert some more drama into the plot.