It's so carefully studied while still keeping a level of subtlety, both in writing and direction. The throwaway lines in every episode reveal more about the characters' personalities, dreams and insecurities than most TV shows I've ever seen, even enormously respected ones like the Wire where we're treated to large obvious (or obviously subtle) plodding "plot point" scenes of Omar being gay, McNulty drinking. Even if you find their lifestyle or whatever uninteresting or irritating, as a tv show it really and incredibly mature debut.
can't wait for season two, a bit worried it won't be as good.