If you ask Markus ‘Notch’ Persson, genius creator of Minecraft, then he’ll probably tell you Minecraft 1.6 was one of the biggest updates and worst to maintain in the history of the game. After 6 different patches, Minecraft Beta 1.6.6 is here and is easily acquired: Just start up Minecraft Beta and it will auto-update. Notch also mentioned that this will be the last update until 1.7 (unless something goes wrong), which is a good thing.
It also seems Notch is getting the long-awaited Adventure mode to debut in version 1.7 — Adventure is a game mode where you complete a series of puzzles or challenges in the Minecraft world without the ability to break blocks (unless you are told to) and materials of any sort are provided (if needed). It sort of makes the game more straightforward rather than the current Survival mode, which forces you to use your surrounding environment to survive, defeat mobs, build mega-structures and invent.
The full changelist of bug fixes and new features is at the source link. Via: The Word Of Notch