I very much like how Luther gets to be terrifyingly good at what he does without pseudo-superpowers. He's a nearly Holmes-level detective without needing the magic dust of genius: he's just/"just" exceedingly knowledgeable, experienced, observant, and dedicated.
That said, Luther's lack of... I don't know, fear? regarding his own violence and unlawfulness is, on itself, dangerous. Frankly, I think the series needs bloody and insane criminals partly to make the acceptability of his methods less ambiguous than it'd otherwise be.
Oh, Zoe. Oh, Mark. It's heartbreaking, how much I like them.
Alice Morgan: Consulting Psychopath. I hope we get to know more of her as a person; it feels at times as if she's undergoing more personal change than anybody else in the series, but we only see her when she's being the World's Most Terrifying Godmother.