Since they first started airing fictionalized series in the early 80's, HBO has long been a safe harbor for misfit shows that would never make it on any other network. Due to their premium subscription model, HBO has always been able to get away with producing shows that would never quite fly on networks ruled by advertising dollars.
Sometimes that's because they're existential voyages into bizarre worlds (Carnivale), sweeping epics across a fantasy world of sin and magic (Game of Thrones), or just plain loaded with gratuitous amounts of nudity and gore (Tales From The Crypt). Their ability to operate outside the limits of traditional television helped HBO kick off the golden age of TV. Although every show cannot be so lucky to work with that kind of creative freedom, there are always shows that try to get away with more than their networks have traditionally allowed.
Some are raunchy, some are bold and some just trying to force a square peg into a round hole. While many of them ending up doing pretty well for themselves considering the situation they are working with, the truth of it is that these shows will always feel like would be better off with that famous HBO logo preceding them. Because it doesn't take sex and violence to make good television, but it certainly doesn't hurt.