You don't pay attention to the story at all, do you? Deathwing has been a puppet of the Old Gods since the War of the Ancients. The Dragon Soul (Also known as the Demon Soul) was specifically created to help free the Old Gods from their prisons. The Elemental Lords were all servants of the Old Gods. The Naga, and Queen Azshara, were transformed into their current state by the Old Gods, and presumably up until now, served them. We saw several Forgotten Ones throughout the expansion. Iso'rath in Twilight Highlands and then Shu'ma and Go'rath in the Dragon Soul raid, and they are chief servants of the Old Gods. Hell, we had two Faceless generals in the Dragon Soul raid. Not to mention that Cho'gall was possessed by C'thun, an Old God.
Yes, the villain of Cataclysm was Deathwing. If you had paid attention to the story at all, however, you would know who the real villains were. The subtext wasn't exactly deep in Cataclysm, either.
As for TBC not being about whoever sent the Burning Legion...did you not play through Sunwell Plateau? Because the guy who sent the Burning Legion is the final boss of it, and by extension, the entire expansion. His name's Kil'Jaeden, I suggest you go read up on him:
http://www.wowpedia.org/Kil%27Jaeden
As for demons and undead not being "new" to WoW, neither were the Pandaren if we include The Frozen Throne. Even if we don't, Chen's Empty Keg is a real quest from the vanilla version of The Barrens:
http://www.wowpedia.org/Chen%27s_Empty_Keg. Even after that, we had the Pandaren Monk pet that was revealed in Wrath of the Lich King.
Finally, stop saying they copied a real world culture. Not only have several of posters already brought up the Vykrul, but several posters have attempted to explain to you that "Asia" isn't a culture. Neither is "Southeast Asia". Until you actually understand that, I will continue to bring it up.