Actually establishing Superman first probably would have helped. Not making him such a loner outcast who seemingly has no relationships or friendships to anyone outside of Lois, and his mother. I mean, Pete Ross, Perry, Jenny, and even Lana Lang were at his funeral, but you really don't get a sense of their loss because none of those relationships were established. Maybe give him more positive interactions with the people he saves, rather then just portraying him as some god in the sky with no personality or connection to humanity. That way you might actually believe the world really is mourning his loss. As is, it's just tough to imagine very many people being too upset the guy is gone.
I mean, Snyder, and everyone else involved with the film would tell you Superman still wasn't fully formed in this movie. So, if that's the case, why do his death? It's just not going to have the same impact that it could have if they actually took their time, and established him in another movie or two.