Look around; there are powerful beings and forces all around. They are also in conflict. If you've seen Tales of the Past, the big danger was Arthas vs. Kil'jaeden destroying Azeroth. We're ready for something even bigger. Let's see who we can get into this apocalyptic battle.
Arthas and the Scourge: The undead are pretty obviously opposed to everyone else. While they were once the tool of the Burning Legion, Ner'zhul broke free and is definitely not letting himself be recaptured.
Burning Legion: They want to kill everything. This isn't complex.
Old Gods: I honestly am not sure what they ultimately desire. In the short term they seem to like chaos and corrupting lesser beings. Over the longer term they clearly want to get free of their prisons. It was indicated in War of the Ancients that they have some interest in taking over the Burning Legion, though I got the impression that it was for conquest rather than destruction.
Titans: They like order and will beat the crap out of anyone disorderly.
Dragons: They're supposed to be guarding Azeroth in the absence of the Titans, but lately they seem to have developed an interest in killing all of us.
Mortals: We're not interesting in dying and will beat the crap out of anyone that gets in our way. Internal divisions would seem to make us weak, but they also keep us ready and as we've seen from the various alliances we can unite when something bad enough comes along. Recently we're busy fighting each other, but I imagine the Garrosh and Varian might get the same treatment as Proudmoore.
So how is this all going to go down, who is going to hate who, and who could possibly form an alliance?
Scourge: Not the Legion. Not the dragons. I doubt the Titans would look favorably on the Plague. But what about the Old Gods? Having surrounded himself with Saronite (old god blood), Arthas may be feeling their influence and might be a bit more accepting of help against an outside enemy. Also call me crazy, but working with the mortal races does not strike me as entirely out of him imagination. In some strange way I think the Lich King may be simply trying to build up an army to be ready for the Legion.
Burning Legion: They want the Scourge back under their control. The dragons and Titans are their biggest enemies though, with even Deathwing ready to fight them, though for his own reasons. They might attempt to work with the Old Gods. The mortal races? I already stated our opposition to dying.
Titans: I'm afraid of them. How will they react to Azeroth being filled with beings who have been pretty severely infected with the Curse of Flesh?
Dragons: They keep deciding to kill mortals with the exception of Alexstrasza and maybe Ysera. But... I predict that Ysera is being corrupted even now and before long will be trying to kill us. Alexstrasza will take longer, but how long before she decides that the only counter to death is undeath?
Mortals: We like to fight, but once someone takes care of Varian I'm sure we'll be able to work together long enough to not all die. Then we'll go back to fighting. But there are always those who hunger for power at the expense of others...