I just wanted to add something that I haven't seen mentioned yet...
The show's co-creator and executive producer is long-time Hollywood writer John Milius, credited with many a memorable screenplay, as is mentioned on HBO's site:
John Milius' (co-creator, executive producer) credits include "Rough Riders," "Clear and Present Danger," "Geronimo: An American Legend," "Farewell to the King," "Extreme Prejudice," "Miami Vice," "Red Dawn," "Conan the Barbarian," "1941," "Apocalypse Now," "Jaws," "The Wind and the Lion," "Magnum Force," "Dillinger," "The Life and Times of Judge Roy Bean," "Dirty Harry," "Evel Knievel" and "The Emperor."
Milius has penned quite a few screenplays including television episodes, (Miami Vice in particular) and has ran the gamut of content - from politics to violence to comedy.
My guess is that he's the main guy responsible for such great writing, believable characters, and high production values, and I tip my hat to him.
(P.S. - great forums you have here, wish I would have stumbled upon them a long time before now!)