From my experience when you find a movie wanting you can't help but say ''this is how it should have been...''. Some people fill in those blanks with fan fiction stuff, others turn to comparable movies when they are available (like this one)
I've done that myself, I've occasionally thought 'why didn't they do this or that' after a film when it didn't feel fulfilling. The difference here seems to be some people are going in with a preconceived notion of what the character of Superman is suppose to be and that is clouding things more and elevating the flaws further. Suddenly the issues get blown out of proportion because it's not what they wanted, which is ironically the same thing critics blast fanboys over when it comes to changes. The film should be critiqued for what it is, not what it's suppose to be, and I think that's the issue coming through.
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