Notch mentioned back on Monday that the Minecraft 'Adventure Update' was going to offer a significant upgrade to the game experience, and that it has subsequently been pushed back to version 1.8. Version 1.7 of the game is due out very soon and simply adds pistons and a few bug fixes, with the large-scale 1.8 update coming after "however long it takes."
"Since 1.8 is still a significant time away, we're going to have to stop being as secretive about it as we have," Notch says on his blog. "We'll provide information about it as we go, but we'll keep actual details secret."
So far, Notch has said that the update will significantly change the way the game is played, making exploration and combat more rewarding for players. He's confirmed the existence of "new complex terrain features, at least one new mob, some interesting new combat mechanics, a new lighting engine and some experimental new gameplay ideas." Soon to be added are "more interesting farming, bigger incentives to explore and NPC villages."
Some of Notch's wording suggests that there may also be an experience and levelling system incorporated into the game, but that it won't be incorporated unless it proves to be fun. There's still no word on the proposed standalone roguelike-like Adventure Mode mentioned on the Minecraft Wiki.
The team at Mojang's focus as Minecraft's November release draws nearer is to flesh out the gameplay beyond simple exploration and building. At the moment, the game is a lot of fun, but ultimately directionless besides goals that the player sets for themselves. After the recent Achievements system gave new players more of an idea how to get started, it sounds as if the Adventure Update will be the first step towards making the game a more "complete" and satisfying experience.