i think that a "darker" superman would be awesome, and by "darker" i dont mean him, but the world around him. make the world gritty, make it filled with crime, with governments that lie, with corrupt politicians. you can make a darker superman movie without changing the character, put him in some real life or death situations, where he does the RIGHT thing no matter what. show him agonizing over decisions....
show him trying to influence the world as best he can and show him having setbacks....some peaco on earth kind of sidestory would be interesting...i think you can use superman to make people think about problems on a global scale. use superman to deal with problems that affect us personally, as far as charity and such, while also making his earths supreme protector...
for me when they say "darker", i dont get my undies in a wad, i think that they want to take supes in a more relatable experience, well at least thats what i hope for