Whenever you translate a book to TV or a movie, there's danger that it won't translate well. However, there are times when it translates beautifully, and it seems that is more often the case when books become a series than just a one-off movie. More time for plot and character development and the nuances that are in the book.
That said, translating A Song of Fire and Ice into an HBO series is total genius. There are three reasons for this:
1. Episodic format fits well with the structure of the books. The books switch between settings and POVs, essentially chunking the epic novels into smaller bites. The show does the same.
2. TV trims some of the bloat. Jenny's got an upcoming post that will address where some of the bloat creeps up in the books, but the jist is that there is a lot of description of mundane details. Part of it is necessary for world building, but there's a lot about small details of dress, accessories, food, family ties, etc. When you change the format to a visual one, you're ditching long descriptions of how things look and letting people just see it.
3. It's easier to keep track of characters. Martin has oodles of characters. They can be hard to keep track of. But, if you tie that character to an actor, suddenly you've got a face, a voice, etc. that remind you who people are. You might not remember everyone's name right off, but you know that one guy is Sean Bean.
Not all stories translate well to the screen, but this one translates fabulously. Whoever first said, "Hey, let's make it into TV" has totally earned the tons of cash he/she is no doubt raking in now.