After watching the finale of its second season, I'm ready to stick my neck out and say that Dexter is the best show in the history of television. As an aficionado of plot, I am in awe; but it excels on every other margin too: dialogue, performances, theme, even music.
As I see it, Dexter is at root a re-telling of Dostoyevsky's Crime and Punishment. The protagonists in both stories willfully transgress moral law by taking life. In Dostoyevsky, of course, Raskolnikov's action fills him with guilt and self-loathing. In Dexter, we see the alternative: Dexter embraces his actions, and thereby condemns himself to live a lie - to be himself with no one but his victims. I know of no other work of literature that so powerfully explores this theme of isolation that flows of out the need to live life wearing masks.
Dexter is also a deep work of political philosophy. All of Dexter's victims are murderers. His procedural safeguards are more stringent than those of the criminal justice system. So why would most of us condemn his vigilantism? Doesn't it just boil down to status quo bias - the system that we have is better because it's what we got? Or perhaps it reflects statist quo bias - we can trust government, but not individuals... even though government employees are individuals.
I could go on. But for a show this good, spoilers would be a crime. All I can say is that Season 1 is available on DVD, Season 2 just finished on Showtime, and Season 3 is coming in 2008!