Perhaps have the humans launch an attack wave against a Horde base where they simply mow down everyone in front of them. Think something like a Roman Legion formation; proper coordinated not much is going to stop that.
For the Night Elves I really think bringing Maeiv into the war would be a good way to bring the savage back into the race. Have an Alliance player start a quest to deal with a group of Horde officers. On the way the player is attacked by elite Horde ncps, who die before reaching the player. When the player confronts the Horder officers the general laughs that the player is outmatched. Next second, the officers are dead and the general has been reduced to ten percent health on knees. Maeiv then appears standing over him and offers to let the player take his head.
The dwarves need the Dark Iron clan to do what they do best; build big, mean, giant iron golems. These golems are then unleashed against the Horde with orders to kill any and all non-Alliance. Unfortunately the golems are a bit too simple in their orders and begin attacking the Pandaren, forcing the Alliance to stop them before things get worse.
Something I think would work good for the worgen, play on their similarity to the English and have them take a page out of Robin Hood's book. As a Horde supply convoy is traveling through a forrest the worgen strike (actually this would work well for the Night Elves too :).
The gnomes need to fight the Horde and not be seen as a joke. Perhaps have the Horde deploy a major weapon, or two, against the gnomes and the gnomes turn this weapon around and use it on the Horde. Simple, clean, effcient, and deadly.
It is hard to think up a dark story for the Draenei that won't go against what is established for them. So instead have the Draenei do something that wil either infuriate or frighten the Horde without being dark. The Forsaken capture a large number of Pandaren to use for forced labor. A group of Draenei Vindicators go to rescue them. These Draenei walk, not charge, walk into the Forsaken base. The Forsaken attack them and the Draenei defend themselves but they do not kill a single enemy. The Vindicators rescue the captives and walk out of the base. A Horde soldier demands to know why the Draenei wouldn't kill them. The leader of the Vindicators turns to him and says, none of you Horde are worth it. And walks away.