I don't know if it's as "grimdark" as some reviews have made it out to be (it probably isn't) but i dunno if you could blame them for being jarred by such a change.
Even if you just put the Donner films aside, hasn't most Superman media been-for lack of a better term-"sunny"? Not talking about comics here, just other adaptations.
There was the old George Reeves show, Lois & Clark, even the DCU show. I've never watched smallville so I don't know what kind of tone it went for but there's seems to be a history of Superman adaptations where "dour" or "no fun" aren't words you'd associate with them.
Again, that assumes what some have said about the film are accurate.
I can understand where they're coming from. For better or worse, the Donner movies are what people think of when they think Superman. FatboyRoberts talked about it a bit on that podcast he was on (great listen by the way). I think for Superman fans, hearing those complaints can be tough because we really can't relate; the comics have done serious and "dark" Superman before. The character has grown beyond just being the sunny, goody good - make no mistake, he's still a really good guy - over the years. Even the DCAU Superman had more of an "edge" than the Donner version.
My issue with it is the complaint is that it reeks of judging the movie based on what you want it to be rather than what it is. If the film has pacing issues or weak performances, call it out. I'm not against there being any criticism at all. But saying it sucks cause "it's not like the movie I saw as a kid" isn't really fair.
In short, I'm sympathetic, but I really would rather people learn to divorce themselves from what they think Superman - or any character - should be and allow for other interpretations. In Superman's case in particular, I think we just need this one movie to get people over the hump and become more open to a different kind of Superman. The sequel, assuming there is one, will probably be better received. In this regard at least.
What wrong with that? Like I said, it does nothing to affect my enjoyment of the film, since my opinion trumps all.
There's nothing wrong with taking the opinions of others into account.