I watched DVDs of most of first season. At first I was repulsed because I took it to be just another instance of pushing the entertainment envelope in the direction of cultural desensitization to foulness, violence, depravity, etc. But, I have to say it's really well done, having the close involvement of the books' author, GRR Martin. I really think he understands common human nature, including psychopathy, intuitively -- and this is rare to see in entertainment, in my experience. Almost *everything* in it is about deceit, lying, manipulation, thievery, greed, conspiracy, revenge, brutality, murder, and seemingly every base expression of self-service such as intemperance, infidelity, abuse, incest, pedophilia, cruelty, etc. The dialog is raw and revealing and no aspect of human behavior, no matter how ugly, seems to be out of scope. Being so open and plain and accurate about human behavior is surprising.
I agree that the sex and violence is gratuitous and overdone, but that's for shock-value, for ratings, for those who respond to nothing more subtle. But this lack of subtlety also extends to the characters' motivations, and that's what makes it most interesting to me. It is naked, real-world humanity on display, revealed in high-quality dialogue. Power is everything. That means money and brute force, and the show makes this obvious. Everyone motivated by power controls others either by threat, reward, deceit or a combination thereof. Hey, it's the real world, precisely! Setting the story in a fictional realm actually makes it easier to see all of this, I think. Although the average viewer will probably take it just as fantasy and not connect it to people being exactly the same way today -- we hide these things so well in "civilized" society.
One of the "noble houses" represents pure, genetic psychopathy. Everything they do is about scheming, manipulation, "anyone not us is an enemy" -- and you see them reinforcing these concepts to each other and their next generation. The methods of their manipulations are plainly revealed by their speech. It's eye-opening. For those characters that are shown to possess some conscience and true feelings despite the brutality of their society, they are utterly still under the sway of that societal programming -- which is described as tradition, custom, habit, ritual, "the way it is," etc., and they engage in the same power games to survive - threat, intimidation, fear, reward. Most of the other characters are basically "fence sitters," taking the easy road of loyalty to whatever power currently benefits them most. So, this fictional realm looks pretty much like the real world to me, while most entertainment reflects a misconception of reality, either by mistake or by design.
I nearly forgot to mention -- as others have -- the show has a thread of hinting at something like a semi-supernatural apocalypse bearing down on the whole world.