It's not the premise that makes Dexter so good, it's the execution (excuse the horrible punning). The stories are played out over a whole season, in such a way that they're highly involving. Earlier on, particularly, there's some strong psychology in the writing, and you're encouraged to get into the guy's way of thinking, and make your own judgements on his mental state. It's also brilliantly acted for the most part, and there's a sheen of quality and no-compromise attitude throughout the whole production.
When I first started watching, my initial skepticism was forgotten by the end of the inspired intro sequence, which in many ways is a microcosm of the show's approach.
