The Abyssal Maw is the elemental plane of water, one of the four such planes created by the Titans to contain the elemental servants of the Old Gods so that their own creations could thrive on Azeroth. Deathwing's prison break will break the walls between the elemental planes and Azeroth, with the Abyssal Maw being no exception.
But this is where the lore gets very interesting. It is said that Neptulon the Tidehunter, ruler of the plane of water, possesses a powerful weapon that allows the wielder control of the very seas of Azeroth. In the expansion, many interested parties, both malevolent and less malevolent, will be after this weapon, the Horde and Alliance included.
Of course, as anyone who watched At World's End could tell you, giving mortals control of the seas via supernatural means really never ends well. But the idea that the Horde and Alliance are now vying for dominance over the elements themselves gives a very intriguing glimpse into the depths of the coming conflict in Cataclysm, and definitely leaves me hungry for more.
The preview includes a collection of screen shots, although they are all (to the best of my knowledge) recognizable from BlizzCon 2009. Still, it's a new update, and hopefully this may just mark an opening of the information floodgates as we look forward to an Azeroth without Arthas.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.