Kilgrave was so terrifying because he is the villain you are most likely to face in real life. Fisk might not have any superpowers but unless you work in very specific industries you're not running into him. But Kilgrave... Someone like that can just appear in your life and tear it apart. Sure, maybe you can't actually be forcefully compelled via magical mind control but anyone who has ever lived through series emotional/mental abuse can tell you that at points it sure as hell doesn't feel like you have any option but to do as you're told. And they captured it so perfectly, having Kilgrave driven not be some desire to be evil but from this simple obsession to control Jessica. He did everything the way you would expect it in real life: he said he loved her, he made the abuse something done for her own good to show her she needed him, he tried to destroy her connections to the outside world, to force her to be with him not because he "made" her but because it's something she "wanted".
They used his super power in the best way possible because if you took it away you still have the same basic character. His power worked as a metaphor for, well, power and privilege taking very real traits and exaggerating them. He controlled people the same exact way real life abusers control people and the only difference is that no one can point the blame at Jessica by saying, "well why did she do what he said if she really didn't want to be with him? If he's so bad, if it was rape than why did Jessica let it happen?" They removed that argument by saying, basically, magic science. She literally couldn't so now you have to accept that this terrible thing happened to her that she couldn't stop and you have to believe she was the victim. They made Kilgrave the inarguable villain which is actually surprisingly difficult when you're telling the story of a survivor of this sort of abuse who isn't the perfect victim. Kilgrave's character allowed Jessica to be a total mess without people lying the blame at her feet. The writers and Tennant absolutely nailed it. Everything from the way his presence is felt even in his absence to the later episodes when you start to see just how delusional Kilgrave is and you start to realize, oh god, he isn't doing this out of some mustache twirling need to torment the good guy. He doesn't think of her as a good guy at all, he thinks of her as something that is his. I disagree with the idea that you needed lesser villains or that Kilgrave's purpose in the narrative dragged. He makes the whole series work.
