The next update (version 1.3) to Minecraft is set to offer a fundamental change in how the game works in its entirety. Notably, the single-player version of Minecraft will no longer exist on its own and instead live as a shell on top the multiplayer game.
Mojang says that this fundamental adjustment to the game will make the game a lot easier to modify, as the company will not have to release separate versions of the modding API each time they update the game. Additionally, if a bug is fixed in either single player or multiplayer, it'll be fixed for both without the developers having to work on each build separately.
Minecraft 1.3 sees plenty of new additions, including a new chatting screen, the ability to share your single player worlds with your friends in multiplayer, a trading system that allows you to conduct commerce with villagers, emeralds and emerald ore, and the possibility to write books in Minecraft and leave stories for others to read.
Additionally, there's a new tripwire system so you can set up deadly traps and a variety of contraptions. There's also new stairs, new half-slabs, cocoal plants, tweaked dispensers, leaves, cauldrons, levers, gravel, pressure plates, cookies, buckets, boats, minecarts, ice, furnaces, and the ability to get magic orbs from mining and smelting.
All that's missing is the kitchen sink.
Source: Mojang