5. The Mandarin
This one goes without saying, so let's get it out of the way first... So it turns out Ben Kingsley's character was just a drunken, bumbling actor playing the Mandarin and that the 'real' Mandarin is Aldrich Killian (Guy Pearce). Now, I'm not saying that a reinvention of the Mandarin was a bad idea - it certainly isn't, because a martial artist tearing Iron Man's armour with his bare hands wouldn't really work in the Marvel Cinematic Universe - but Iron Man 3 didn't really reinvent the Mandarin at all. It could have been done so much better. What Iron Man 3 essentially did was give the Mandarin name to a completely different villain from the comics, which ultimately means that the Mandarin doesn't really exist in the MCU. It would have been one thing to change the Mandarin character - Perhaps by giving him some tech to rival Tony's, such as making his rings tech-based instead of alien-magic-based to make him fit in more with what we've come to expect in the MCU, but what has actually happened is that the Extremis-enhanced Aldrich Killian - a villain who is memorable enough from the Extremis comic book arc to stand alone in a movie - has taken the name 'Mandarin' without taking any other elements of the character. This has basically denied the audience the opportunity to see a real on-screen interpretation of the villain who is essentially Iron Man's greatest foe. It's like denying Superman movie viewers the opportunity to ever see Lex Luthor or General Zod. It's like having a Batman trilogy without utilising the Joker. It's like giving Venom eight tentacles and calling him Doctor Octopus. As great as the film was, it leaves the audience (comic book fans in particular) feeling cheated with regards to the Mandarin.