Note: I like "official" threads
Note #2: Make this sticky
Anyway, to start things off, the link is
http://www.blizzard.com/war3x/
It's updated often with new info, so check it occasionally. So far, two new heroes have been announced: The Blood Mage, and the Crypt Lord (both are shown on the site).
Also, this is copied from the main page:
One new Hero per race, each possessing powerful spells and magical abilities
A host of new units, each equipped with new abilities and spells, giving players the opportunity to create diverse strategic and tactical forms of combat
Player-built shops, unique for each race, equipped with items carefully designed to improve and aid the units of every race
Neutral Heroes, available for recruitment by all players, that can supplement and strengthen a player's army with all new spells and abilities
Expanded multiplayer options over Battle.net® including multiple new game types, clan and tournament support