A lot of speculation --and the underlying assumption-- is that Deathwing will be the killable end boss in Cataclysm. End boss? Likely. Killable? I'm not so sure.
In Wrath, we got the chance to destroy one of the Dragon Aspects, Malygos. He wasn't even the end boss in the expansion, just the end of one of the story chains in Wrath. From this, we can go in two directions: either Deathwing is more powerful than Malygos ever was, or Malygos isn't truly dead. (A third possibility is that Malygos got nerfed by Blizz in a patch release and we caught him on a good day, but you never know.)
If Deathwing is truly more powerful than Malygos --and by extension, the other Dragon Aspects-- then there's very little stopping him from killing Ysera and Alexstrasza. Also, if he is truly powerful enough to break the world, perhaps the best we can hope for is to beat him back and shut him out of Azeroth once more.
Now, if Deathwing is equivalent in power to Malygos, and his retreat from the other (pissed off) Dragon Aspects in the Second War implies as much, then the downing of Malygos (even with the Life Binder's aid) might not have been the end of him. It won't be the first time that someone once thought dead makes a reappearance in a Fantasy environment. ("What, Sauron is back again? Just how many lives does this guy have, anyway?")
Another possibility is that Deathwing isn't the true last end boss in Cataclysm, but someone --or something-- else. Possibilities abound: another Old God or two, Azshara, Elemental Lords, or maybe even Sargeras.
I keep bringing this up, because I think it's going to be a very important theme for the Horde in Cataclysm: the Cataclysm itself won't do as much to the Horde as the fight with the Alliance will.
From what I've seen so far, the Horde region that is hit the hardest will be The Barrens. Sure, Cataclysm splits the region in half, but the greatest impact is on the Alliance's push into Southern Barrens and the hasty construction of barriers at the entrance to Mulgore. Shenanigans with the Grimtotems will impact the Horde/Alliance war and will change the face of the Horde forever. Ashenvale is going to change significantly as well, given all of those Horde victories in Warsong Gulch. And that's not counting the Forsaken turning their attention to the Greymane Wall.
Speaking of Forsaken, whom do they turn their ire to? Arthas is dead, and the Scourge will be (theoretically) more contained than before. (Gee, I have no idea why... The Lich King is dead, isn't he? At least that's what the official stories say....) So, who gets the brunt of the Undead's vengeance? Or, will this lack of a major enemy be a major theme in the Forsaken story?
Although technically they are part of Outland, what about the Draenei and Blood Elf starting areas? Will there be changes in Eversong and the Ghostlands given the current state of the Scourge lore? How will the Draenei start off in an "unbroken" world that doesn't acknowledge that they were around for the Cataclysm?