Everyone has at least one alt in WoW. It may be the bank alt you send all your items to while questing away from a capital city. Some of us have one of every class, or multiple alts on different realms to play with friends on a different server. Cataclysm is going to change the way we play our alts forever, though. We will have goblins named with every variation of "Gringotts" used as bank alts. People will name their worgen after Twilight characters or make names that mocks Twilight.
But the two new races aren't the only things happening come Cataclysm. We are getting new race/class combos. Roleplayers will have new characters they can roleplay or add the new class/race combo into their character's story. While Blizzard has yet to give us gnome paladins, our little friends can now be healers, which gives us the chance to take 24 of our gnome friends and knock on Deathwing's front door.