As a big fan of the books, I find it impossible to evaluate the television series truly objectively, and I can't write a review of this show that is completely newbie-friendly. There will be mild spoilers for all five books—in the sense that implying characters aren't dead yet is a spoiler—and I will assume you are familiar with the books.
Game of Thrones has several things going for it besides the strong source material. The production value is pretty incredible, and the combination of on-location shooting around the world (from Iceland to Malta!) and seamless CGI results in the sensation that the show was actually filmed in Westeros and Essos. Ramin Djawadi's score—beginning with the epic theme music—is evocative of the different locations and never overpowers the scene. The casting is fantastic for the most part, and with 25 actors in the main credits by season two, that's an impressive feat.
There are several standouts in the cast. Sean Bean really gets to the core of Ned Stark as a man of honor. Peter Dinklage is an absolute delight as Tyrion, but he does more than deliver good wisecracks and slap Joffrey; he completely sells him as a broken man who has put up a facade of not giving a fuck that everyone hates him. And some of his best scenes are with Lena Headey as Cersei, who takes one of the most loathed characters in the book and—with the help of the writers, who conveniently cut out a lot of her worst actions and portray her in a more sympathetic light—makes her one of my favorite characters, a fascinating woman in power, a fascinating mother in power. Maisie Williams is a real find, the perfect Arya, able to hold her own in scenes with much more experienced actors. Honestly, I could write several paragraphs on how great the cast is, but suffice it to say: the cast is great.
The most interesting and controversial topic to discuss regarding the show is, of course, the ~*changes*~. How do they go about adapting these long, involving novels told from multiple POVs? Well, for starters, they ditch the POVs as dictated by the book. What this means is that there are many scenes in the show that are completely new, some of which are between two characters whose POVs we never get. And, more often than not, these scenes are the highlights of each episode, as they allow the writers to fill in gaps and explore character relationships, as in a wonderful scene between Cersei and Robert, which no one in the book would have been witness to and which neither character would have talked about to anyone but feels perfectly in-character for them. Not to mention the fact that the show gives us scenes between Varys and Littlefinger, the two most Machiavellian characters in the series and the two characters GRRM has said he could not give POV chapters to because they simply know too much. These scenes feel like a gift to the fans of the books, but they also enhance the show as a whole.
In the first season of the show, it was these bonus scenes that were the most noticeable changes. The show generally hewed close to the book, almost to a fault, which sometimes sapped the series of true narrative momentum during the early episodes where characters had to be moved around to their proper locations. Jon Snow's characterization suffered, but on the upside, John Bradley as Sam was adorable as all get-out. The show excelled at the Big Moments, however, usually saved for the ends of episodes, and it was exciting to see these events play out onscreen. And the climax of the season was so brilliantly executed that even though I knew what was coming all season, it still brought me to tears.
In the second season, however, showrunners David Benioff and D.B. Weiss begin to take a lot more liberties with the source material, and, again, more often than not, these changes improve the quality of the show greatly. They begin to write the show as a season of television rather than an adaptation of a novel, and it is here that it begins to approach a semblance of comprehensibility for a newbie. Cliffhangers now come from scenes implied in the novel but not shown, and it heightens the tension. Characters who didn't interact in the book now have scenes together, and it's magic. Benioff and Weiss are also writing the show with knowledge of the whole series in mind and smartly highlight certain characters and conflicts that will be important later on. Theon benefits the most from this treatment, as his role in season one is increased, and he is portrayed more sympathetically in season two (basically everyone—except Joffrey—is portrayed more sympathetically; even Tyrion gets to have some of his nastier side reduced, as if there was that much danger of his not being the fan favorite anyway). We get to see Margaery Tyrell, who is somewhat of a cipher at this point in the books, as we know her to be later on in the series. They do make their share of boneheaded moves, however, sometimes making mystifying changes that fuck with characters' motivations.
I was glad that I waited until I had read all the books before watching the series, since it took so long that I had forgotten most of the details of the first two books and could simply appreciate the story I was told onscreen with slightly different versions of the characters I had been reading about. The show gets so much right and excites me for various reasons that I can forgive its flaws. They have brought the books to life in a way I didn't think possible, and I hope the show only continues to improve.