First off the Ravine's and the new Lighting update will be added in 1.8.
New things that are being added are:
Giant mushrooms and npc villages (randomly generated villages with Ai people)
Randomly generated dungeons with what seems to be 4 new block types that look like Stone brick, cracked stone brick, mossy stone brick and iron bars
New bigger biomes (i assume that there will be a new biome or two as well not just bigger biomes-may be wrong-)
A new (scary) mob
Sprinting, Notch hasn't decided on the drawbacks of sprinting yet.
More farming options (hasn't given details on what)