I agree that Malekith is the biggest villain misstep so far, but I wouldn’t go so far as to say that he’s the only lackluster villain in Marvel’s lineup.
I thought Ronan was almost as underdeveloped as Malekith.
I thought Mads Mikkelsen was pretty underutilized in Doctor Strange. Kaecilius is not a bad villain, per se. He’s just disappointing because he never lives up to his full potential. Given what we do know about that character, he could have been a much, much better villain if he’d been fleshed out a little more.
And yes, you make a good point about Darth Vader not being very fleshed out. Hans Landa from Inglorious Basterds is another magnificent villain who isn’t fleshed out very much. And yet… I do still find myself wishing that we’d gotten another scene or two with Kaecilius.
Hela actually is another villain I found disappointing. I really dig both the mythology/lore and the visual design behind that character, and Cate Blanchett was a delight to watch. She was clearly having a lot of fun with that character. There was still something disappointing there, though, and it’s hard to identify just what it is.
If I had to put a finger on it, I’d say that it’s the fact that she’s up on Asgard by herself for most of the movie, and there are no characters we care about anywhere around. She eliminated the Warriors Three in about two seconds flat, and the rest of her time in the second act is spent slaughtering an army of Asgardians the audience has no reason to care about.
I think it was a huge mistake killing off the Warriors Three that quickly. If they had been kept alive a while longer, only to be killed off in the climax of the scene where Hela lays waste to the Asgardian army, that would’ve made for a much more impactful scene.