Great writing, great acting, blockbuster quality on a cable channel. They have long uninterrupted seasons that don't ruin plot points by drawing them out too far. They have the entire first and second season available on demand because they know that letting new viewers catch up is more valuable in the long run than DVD sales.
Probably the only series out there that is really worth the time.
Season 3 is based on the first half of "A storm of swords" which is highly regarded as the best book of the series. They hardly have to try this season and next. Everyone who's read the books can't wait to see your faces on the last 3 episodes.