George R.R. Martin is still underrated, even given his success. He wrote for television for a long time and structured his novels more like seasons of TV, with each chapter serving as an episode. This is part of the reason for their popularity. Martin is also a once-in-a-generation genius of character, structure, and plot - a self-evident truth that is rarely spoken aloud because genre fiction is "not literature."
The creators of the GOT show really never even realized that they were adapting something carefully, painstakingly honed by a genius craftsman. They really figured they could just keep throwing in dramatic, violent twists and we wouldn't know the difference. In that light, it's shocking that they even made the first seasons as competently as they did.