Blizzard has announced the initial roster of heroes that you will be able to play as, when the game launches. The heroes range from three of their major titles, but I imagine that they will keep on adding new heroes once the game launches.

World of Warcraft

• Arthas
• E.T.C. (Tauren from the WoW rockband "Level 90 Elite Tauren Chieftain" or "L90ETC")
• Falstad
• Gazlowe
• Illidan
• Malfurion
• Muradin
• Stitches
• Tyrande
• Uther

Starcraft

• Abathur
• Kerrigan
• Nova
• Raynor
• Sgt. Hammer
• Tassadar
• Zeratul

Diablo

• Barbarian
• Demon Hunter
• Diablo
• Tyrael
• Witch Doctor

Source