Since this has popped up quite a bit recently, I thought I'd go ahead and share my theory of the WC3=WoW class connection, and how it has shaped the game since Vanilla. Hopefully this thread will provide information about how Blizzard theoretically structures classes, and implements them into the game we all know and love.

Here are the hero units, and the classes that pulled abilities from them. The number indicates the number of abilities pulled from that hero.

Blademaster: 2 Mages/Warriors
Shadow Hunter: 3 Shaman
Farseer: 4 Shaman
Tauren Chieftain: 3 Warriors/Tauren Racial/Shaman

Archemage: 3 Mages
Blood Mage: 3 Mages/Warlocks (Drain Mana has been removed from the game)
Mountain King: 3 Warriors
Paladin: 3 Paladins

Night Elves:
Keeper of the Grove: 4 Druids (Thorns was removed from the game)
Priestess of the Moon: 2 Hunters/Druids
Warden: 2 Mages/Rogues
Demon Hunter: 4 Rogues/Warlocks/Priest (Mana Burn was removed from the game)

Death Knight: 4 Death Knight
Lich: 3 Death Knight/Mages
Dreadlord: 4 Warlocks/Death Knights
Crypt Lord: 1 Death Knights

Naga Sea Witch: 2 Mages/Hunters (Mana shield was removed from the game. Replaced with Incanter's Ward in MoP, Concussive Shot=Frost Arrow)
Dark Ranger: 3 Hunters/Priests/Warlocks (Drain Life was changed into Harvest Life in MoP)
Brewmaster: 4 Monks
Beastmaster: 1 Hunters
Pit Lord: 3 Warlocks
Goblin Tinker: 0
Firelord: 2 Warlocks
Goblin Alchemist:0

Hope this helps. If you have any questions, feel free to ask.