Its not adventure mode, but an adventure update. Notch has planned an adventure mode, where you can craft tools and weapons, and explore other players created worlds, but the person playing the world couldnt place or destroy blocks. Like making an"escape the city" map for others to escape without building over walls and stuff.
Now the adventure update is different. It provides more incentives for players to explore the land, and fight monsters.
Some things notch has confirmed about adventure update 1.8 :
1.npc villages
2.A new enemy(possibly the un-used silverfish texture found in the "mobs" folder of minecraft 1.7.2
3.giant mushrooms
4.underground and above ground ravines
5.new lighting system
Here are some speculated changes :
1.Skyland(or the dreamworld)
2.a leveling up system and experience orbs
3.a new underwater enemy(possibly)
4.he will probably remove herobrine again XD
Wheatly: Umm... False. Im going to have to go with False, although to be quite honest, I might have heard that one before.