As previously reported the update will add variable signal strength for redstone, a new "capacitor" block, a daylight detector, fireworks, and Nether ore for the capacitors.
Other new additions and pre-release details were announced on the Mojang blog. The update will also add a hopper and a dropper, both mechanisms for moving items around. An Activator Rail will trigger TNT minecarts. New building materials include a block of redstone, nether brick and ore, and quartz.