Lots of good responses to this thread, but some things are thoroughly overlooked... I've voted Kalimdor, and here's some additional stuff that haven't recieved enough attention.
Forces of AQ - raid-bosses aside (C'thun vs Algalon vs Sha of Fear vs LK vs etc etc...), the army that was once locked in AQ and got released at the sound of the gong is probably only comparable to the Scourge and maybe the Mantids. However the Mantids of Pandaria have been held in check by pandas while the forces of AQ required a combined effort of all "good" races including the Bronze Dragonflight - keepers of time!.
Elemental planes - Pandaria might be good fighters, but how will you engage the other continents without a wind to power your boats? The elemental plane of wind is located in Kalimdor (Throne of the four winds or whatever it's called - and Thunderaan ofc!). How will you burn anything when the elemental plane of fire is also located in Kalimdor(Mount Hyjal) - MC is just a "temple" for Ragnaros. Fire will also be one of the only things that destroys undeads, making Kalimdor capable of dealing with the scourge (which might otherwise only be possible for the Lightbringers of EK).
So in conclusion - Kalimdor will win because of...
Forces of AQ
Elemental planes of fire and wind
Malfurion/Thrall combo
Bronze Dragonflight - keepers of time! (seriously, how will you fight someone who controls time? - they do have their counter-parts in Wotlk, but we all know who won those encounters, right? :P )
Goblins are much more suicidal than their gnome counterparts, thus they have a higher chance of making an ultimate doomsday device.
Corruption of the Emerald Dream - if this could be harnessed into a weapon, it would be hard to see anyone countering it. Lore-wise only druids have done so, and all druids are tied to Kalimdor (Lore-wise - I know EK-races can pick druids but you have to admit it suits the Kalimdor-races better - and Moonglade is in Kalimdor as well).
The only thing Kalimdor lacks is maybe the power of arcane magic. EK has Medivh/Karazhan/Sunwell and Northrend has Kirin Tor.
---------- Post added 2013-02-07 at 04:09 PM ----------
I'm fairly certain worgens would count as part of EK, so that might be a bit wrong
