The biggest change comes to the entire single-player mode, which will work as a shell on top of multiplayer. This assures a smoother multiplayer experience, lets you share your single-player worlds with other users on a local network, and allows you to use certain commands in single-player with cheats enabled. This will also make it easier for Mojang to fix bugs, since the company won't have to fix them twice-over for single- and multiplayer.
Other features include emeralds, ore, and a trading system to buy items from villagers. The update will also add the ability to write in books, new terrain features with a "bonus chest" to start the game with a boost, a trip-wire for traps and contraptions, magic orbs from mining and smelting, and a bunch of new or tweaked items.