7. Ronan The Accuser
As great as Guardians of the Galaxy was - and believe me, it was great - the one gripe that fans had with it was how badly it handled Ronan The Accuser. He was a villain with all of the potential in the world and the movie completely wasted him... and then killed him, meaning that the whole MCU wasted him too.
Lee Pace was a force-to-be-reckoned with on-screen, bringing the character's stoic presence to life with ease. He was commanding in all the right ways and yet held back when needed, only letting Ronan's aggression out in the few moments that it was required. It hinted at layers beneath the character's drive for power which were never explored. In fact, those acting nuances are the only hints of anything beneath Ronan's hood, because he really was just a power-hungry tyrant who wanted more control. That's just not that interesting.
Ronan The Accuser is a major player in the Marvel Comics whose reputation precedes him, but he felt like little more than an insubordinate general of Thanos' who could never match up to the level of The Mad Titan. His cameo appearance in Captain Marvel added nothing to that already-wasted potential, and the fact that he was killed after losing a dance-off to Star-Lord is just, well, not good.
This character was capable of so much more. I get it, not everyone can be a Thanos. But he could have been so much more than this.