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First off, I am a Superman fan (think he has so much potential) and I only wish the best for his series. So I'm in no way insulting Superman or his fans.
Alright, even as a Superman fan I can't help but feel that Superman is a bland character. You've heard it before; "Supes is lame. How can he be the iconic superhero?" "Batman is SO much more interesting! He deserves more!". But I can't put my fingers on why he's so disliked. HOW he's such a bland character!
I definitely know WHY: Back in the late 30s Superheroes were just fictional characters, mostly meant for kids, that were part of supernatural or extreme adventures saving the World/Universe. Most were made like a loony tunes cartoons and as a result they could be quite silly, inconsistent or badly written (compared to "serious" fiction). For example, Pre-crisis Superman could sneeze galaxies out of existence but never seemed to do that with all his enemies. It stayed like that for decades until Superheroes became more mature, serious and somewhat realistic. Batman made that transition perfectly! If anything it improved his character. Ideas such as a billionaire using gadgets, an orphan setting out with sense of Justice and determination and characters with mental conditions that made them powerful, dangerous enemies could be transformed so well into the new tone! Superman on the other hand, did well but still faced problems. It was difficult turning a Cheesy, God-like man who could pull anything out of his ass into a character to take seriously, so they set boundaries to him; made him weaker, set down what powers he had, even tried to make his story more emotional etc. But he still wasn't quite there.
Most people say "Superman's biggest flaw is not being flawed enough" (I thought up of that line myself. Makes me feel like Shakespeare!) Make any sense? It basically means he's too strong as a character. He can do "anything". He faces no challenge. He will always come out on top and you know it so you don't feel any tension from his obstacles (he always wins), any sympathy from his pain (if there even is any), any connection to his character (most people don't tend to be born unstoppable).
However, I call this BULLSHIT!!!!!!! And it's not just because Superman is actually stoppable, or sometimes looses or has weaknesses (such as Kriptonite, which once again is sorta bland). It's because there are COUNTLESS other fictional character, who face the same "not enough flaws" problem but for some reason everyone loves them! Goku? He nearly always wins and has no weakness (expect magma but that's very rare) and no one complains! Hulk? Some fans claim he's unstoppable (not even true) and feels no pain and that makes him bad ass but when it comes to Supes it somehow makes him "badly written"! Mad Jim Jasper? He's a reality-warper who screws logic and people think he's awesome but Superman does something a bit out of science's reach and everyone feels offended by lack of logic!
Of course it could be something to do with emotion. Bruce Wayne has a lot of emotional bits as a child but so did Clark Kent when he grew up on an alien planet learning to hone powers that scared him. You could say "yeah but AFTER that, Superman faced no problems and Batman did". If you're talking about fights, there are plenty of enemies who gave Superman a run for his money (Doomsday, Darkseid etc). If it's to do with emotion, then I agree that Batman still faces problems to this day (loss of his parents always haunts him or questions of morality) but most are the same thing every time (I lost count of the amount of times his "emotional problem" in the Dark Knight trilogy was deciding to quit or not.). Yet I don't think it's enough to make Superman a disliked character.
Again, it has nothing to do with realism, because nearly all fictional characters are supernatural and don't get this shit.
So my question to you is, HOW is Superman a bland character? Or rather, What would you do to him (if you had the chance) to make him a better character?