Half-Life 2: Episode Two: "we're working hard on putting the final parts together." The early part of the development was spent carefully working on concept art of key areas in the game, ensuring everyone on the team could share a vision of what the final product would look like. A few examples are in the gallery below.
While yearly releases doesn't feel very episodic to us (especially compared to the monthly Sam & Max series installments), it's half the time we waited between Half-Life 2 (2004) and Episode One (2006). Does Valve need to get its act together and deliver at a faster pace, or does a game like Half-Life 2 need time for perfection?