(no real spoilers except for the names of certain battles)
^^Lots of good points that explain why this decision is actually good for the show, as its slowing the story down and likely splitting what was going to be 1 season's worth of material into 2 seasons, similar to Game of Thrones taking book 3 and splitting that material between season 3 and 4.
Or put another way, HBO and the showrunner is turning 10 episodes into 16 episodes.
The article out right says a battle and plot surrounding the battle that was originally going to be in season 2 is being moved to season 3, a change in story structure that the article says necessitates a 4th season.
Without going into spoilers, I know what plot they are talking about so just from a storytelling perspective, this is a great development because I had fears that season 2 would feel extremely rushed....sort of like if Jon fighting the Wildlings in season 4 or Oberyn fighting the Mountain was included in season 3 of Thrones. But that fear has been greatly reduced with this news.