The Pandarens are to be a new playable race, starting at level 1.
They were discovered after the Cataclysm during an epic sea battle between the Alliance and Horde.
A neutral race to play, until you reach level 10, when you must decide, Alliance or Horde ...
More information is available from the Pandaren Race page.

The Monk is to be a new Class playable by both the Alliance and Horde.
The Monk is capable of assuming the role of Healer, Tank or Melee DPS.
The Monk may wear cloth or leather armor, and may use Fist Weapons, One-Handed Axes, One-Handed Maces, One-Handed Swords, Polearms and Staves.
More information is available from the Monk Class page.

Pandaria is a new continent containing six new zones to adventure in, including the Pandaren starting zone.
The Alliance and Horde will start at either end of this continent, and converge at the center.
Further details are available from the Pandaria page.

A new way of creating a unique character, talents are awarded every 15 levels and compliment your class specialization.
For example, a Holy Paladin will get their holy specializations at level 10, and then at level 15 will choose their first talent, from a choice of three.
This specific feature is likely to change more than any other, and more information is available on the talent page.