I don't think there's a conspiracy, but that doesn't mean individual critics don't push the Rotten button because of several factors - including how hyped the film is.
X-Men didn't really deal with anything serious. You can say it dealt with minorities, but it did it very differently in an unbelievable and popcorn fashion. I enjoyed the first two (actually liked most of Last Stand too).
But there always is a question for any viewer as to what their expectation were coming in. Just by reading the reviews themselves, you can tell the problem with most of the critics is the critics themselves - they had bad expectations.
Many of them wished Superman would quip more one liners and that's the reason why it got a lukewarm review. That the Robert Downey effect. But that's not Superman. If you want a Superman movie with zingers then you're setting yourself up to not enjoy it.
Too much action? Avengers didn't have too much? If you didn't criticize Avengers for action a year ago then you're being inconsistent as a critic. Maybe because Avengers didn't ask them to deal with more serious subject matter?
But I saw similar things with some of the lukewarm reviews of Into Darkness too. Those alleged "plot holes" that many critics attacked were not plot holes at all. Indeed, if you actually pay attention to the movie, you understand where it's coming from.
I just find it funny that a lot of these critics get paid for their opinions, even though their opinions are often out of line with a lot of the public at large. Shouldn't they care more about how the public will view the film than the lens they see it through?
There's a combination of things at work here. Superhero fatigue - maybe a bit. Marketing fatigue - probably wouldn't surprise me if some critics lash out because of the marketing effort. Dislike of movies that try to make superheroes relatable (again, Batman is not super).
Yet this is all imposing the critic's outside perspective on the review instead of letting the movie speak for itself. And it doesn't just go for Man Of Steel. You may find critics that may agree with a film's political outlook call it fresh while they knock down one they disagree with. But do those political opinions matter inside the film? I know I can watch a film and enjoy it even if it may portray something I disagree with because I can separate my opinions from the film and enjoy it on its own merits.
A lot of these critics cannot.
I'm not saying it can't happen but there's so many hyped movies that get fantastic reviews so that argument isn't very solid.
I don't see what's unbelievable about the people in the X-Men movies being outsiders as it's mainly used as allegory. I don't know if any movie superhero has invoked more emotion in me than Fassbender's Magneto, and without going overboard with it. It's not like the Batman movies are at all realistic either (there's plenty of plot convenience, unrealistic events etc if we're to point such out).
Some might want something different in the humor department but what's really wrong with that? People want different things. Some critics just wanted more levity as sometimes a movie can suffer if it takes itself more seriously than what works. That doesn't necessarily have anything to do with expectations either, it can just be that they are watching it and feel that something is missing.
Too much action most likely means more action than they feel the movie can carry. You can't just throw action in there, you have to make it mean something as well. That's why Avengers and Transformers are seen as so vastly different (with very good reason in my opinion).
I didn't follow STID, and haven't seen it, but the ST fans I know say that both the new movies have some plot holes, but they still love them. Fans are generally more willing to let things go. That movie got great critic reception overall though.
And it's pretty natural that critics don't like exactly what the GA likes. When I started taking my movie interest more seriously and learned more about the art of making them my taste in movies changed a bit. I'm far less well-read than the more experienced professional critics though.
And in the end you can say the same kind of things about fans. Lots of people here are defending a movie they haven't even seen. Can you imagine how people would react if critics started saying which movies are good or bad before they seen them?
I also saw someone listing RT scores, showing a couple of examples of high scores (The Godfather and No Country For Old Men) and dismissed critics on the notion that he was never going to watch those two movies. That means he said that critics are bad because they like things he's not interested in, and he's dismissing movies that he hasn't seen. The overall attitude doesn't seem unique here.
Judging from this whole thing I think the fans act more irrationally than the critics are, and that's despite that I expect to like MoS.