More information was released regarding 1.8 today. Notch mentions some exciting changes for the Adventure Update, including:
"...new complex terrain features, at least one new mob, some interesting new combat mechanics, a new lighting engine, and some experimental new gameplay ideas. To come is more interesting farming, bigger incentives to explore, and npc villages."
Additionally, Jeb_ has confirmed release of the updated terrain.png and items.png files, so texture pack artists can have their packs ready to go by 1.7's release. He also added that mossy cobblestone has been updated for 1.8, but will not be included in 1.7's release.
We will continue providing information as it develops.
I personally don't care for all the adventuring stuff. I mainly like to build and screw around. Some would tell me to just play classic and shut up, but thats not good enough for me. Classic is ok, but I like building in beta to use all the cool stuff like doors and fences that classic doesn't have.