The limitations of RT's rating system have been much commented upon so it's frequently not the best gauge. But the criticisms I've read so far sound pretty standard for a Zack Snyder film and were why I was worried about him directing Superman - it felt like a mismatch of director and material. Snyder tends towards the morosely bombastic (or is that the bombastically morose?)
That said, the reviews do really sound like people are a tad confused by their expectations and how the movie subverts them. It makes me wonder if this isn't why Superman is such a difficult property to manage on screen. Everyone can say that it's hard to do a Superman movie because he's too good and pure of character with no shades of grey to make him interesting but that always struck me as pure bunk. There's plenty of elemental conflict in the concept of Superman. But I think Superman, these days anyway, may work better in niche interpretations and where people get into trouble is trying to do a version that has broad appeal. I mean, he's a core heroic American character and his heyday was when there was a monoculture he could play to. That is long gone, and while you'd think a postmodern America would embrace a postmodern Superman, there's enough nostalgia at our core for the myth of the grand American ideal that we still want Superman to scratch that itch. Unfortunately, we've all pretty much got different itches for him to scratch and still expect him to be able to be universal - so he kinda can't win.
Which is by way of saying, I do think movie critics may be (haven't seen the movie, so how would I really know?) expecting too much.
Personally I love to see someone go soemwhere unexpected with an iconic character like Superman, so I'm mostly even more intrigued by what's emerging from early viewers. It may not be great, but at least it's taking a risk.