Since 1.8 is still a significant time away, we're going to have to stop being as secretive about it as we have. We'll provide information about it as we go, but we'll keep actual details secret. So, for example, we could say “added two new colours” without saying which ones, or “testing experience and leveling to see if it's fun” without giving details on exactly how it works.
The update is pretty big, and will change a lot of how Minecraft is played, focusing on making exploration and combat much more rewarding, and bringing in a bigger sense of adventure to the game.
Already implemented are 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.