"...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.