Malekith the Accursed:
He was SO freaking boring. There was barely any emotion at all, even when there should have been. His people were destroyed in a war with the Asgardian, does he ever express anger, or sadness, or anything other than robotic blandness, nope. He gets half of his face burned to a crisp, and he has so little reaction to it that I'm not even sure that he noticed that it'd happened at all. He destroys Odin's throne, the symbol of power for his people's ancient/hated enemies, and he just keeps on walking without giving it a second look, and so one. Yeah he was "the Accursed" alright, accursed with a complete lack of personality.
And what makes this even more mind-boggling is that Malekith in the comics has LOTS of personality. He's almost Joker-like in his scary yet also darkly funny bat*** insanity. And then they hired Christopher Eccleston, a guy who's acting style is all about emotion, and had him play this walking sack of blandness. So it was a characterization that didn't fit the character, or the actor.
Ronan the Accuser:
Ronan had similar problems, although he was a big better because Lee Pace got to show SOME personality. I wasn't bored to tears whenever Ronan came onscreen. My big problem with Ronan was that he was just a two-dimensional genocidal nut-bag. Ronan in the comics is SO much more interesting/complex, he has dimension. MCU Ronan, none of that. And since they killed him off, the chances of him getting dimension in the future seem slim to say the very least.
Ivan Vanko (Whiplash):
This villain is really disappointing because he had such potential. He had a great look, his backstory was good, Mickey Rourke's performance was really good, he actually had a pretty interesting/unique plan, and they built him up pretty effectively for the first part of the movie. But after the Monaco fight, he disappears for large chunks of the movie, and when we do see him, he's just stuck in a lab building robots. The movie focuses much more on Justin Hammer who, although Sam Rockwell is entertaining, is no nearly as effective/interesting as a villain. Then we get to the big climax and Whiplash's plan has all of a sudden degenerated into a generic "kill the hero" plot with no real explanation. Overall, he was such a waste of potential.
Red Skull:
Hugo Weaving gives a good performance, but he doesn't get nearly enough screentime. And when he IS onscreen, most of the time he isn't really doing anything. And ironically, I think that the movie didn't let him be evil ENOUGH. RS is supposed to be truly vile and despicable, worse than Hitler. And I didn't really get that in the movie. Also the reason why he and Cap are such good foils is because they're the polar opposite of each other. Each one represents a worldview that the other finds repellent, the complete antithesis of what they themselves believe. The film paid lip service to that idea, but never really delved into it enough imo. RS just came across as another generic bad guy that Cap needed to stop. No different from Crossbones or Alexander Pierce in TWS, except that Pierce was much better-written imo
Overall, Marvel villains (with a couple of exceptions) seem like an afterthought a lot of the time. Like the writers are working on the story and then realize "ohcrap, we need to give the hero someone to fight don't we?" And I don't really agree with this philosophy because I feel like having a good villain not only raises tension/stakes, but makes the hero MORE interesting because they have a truly effective foil to play off of.