Worgen Rogue! What could be more frightening than getting ganked by one of thoses? I am very excited about this announcement.

Just a little info on why Alliance would get Worgen and the Horde get Goblins:

Goblins served the Horde pretty much exclusively before the Third War. Even in today's Azeroth they still favor the Horde, giving them use of their zeppelins. The Goblins that are joining the back up with the Horde are more than likely only a minority amongst the Goblin race (probably under a Trade Prince) and the majority of the race will still remain neutral, lore-wise.

The Worgen allying themselves to the Alliance are more than likely the transformed nation of Gilneas, once one of the seven bastions of Human power. Their ruler, Lord Genn Greymane, pulled their support of the Alliance after the Second War because he refused to spend his nation's resources on keeping the orcs alive in internment camps. He shut his nation off from the rest of the world, and no one has seen the Gilneans in over 10 years. Since then a Worgen infestation has taken place just outside of the Greymane Wall, and it is fairly safe to assume that if one of these beasts ever got loose in the isolated nation, they could easily turn it's majority.

Much of this is still unconfirmed, but the pieces of the puzzle fit together well, albeit with a few missing pieces. We will all find out more about this later this month, I'm sure.