The Mists of Pandaria beta has started, and with it a ton of new information has come out. Not only are people finding out about items, game mechanics, and zones, but people are finding out about lore and what things will finally look like. With that, we have finally seen in game models for the Sha. I would link them or otherwise include them in this post, but my work internet blocks all the sources which could provide it to me. However, let me describe it to you. They look evil, of course; the one I saw looked like a much nicer shadowfiend. However, more importantly, they were black with white accents. I'm sure MMO-Champion would have a picture.
The Sha and the Pandaren are only found in Pandaria. The Sha and the Pandaren are black and white (if we ignore the red pandaren females). The Sha respond to emotions. It seems to me that the Sha and the Pandaren don't just share a common location, but are actually linked.
With Warcraft we have Old Gods and various other evil forces. So I think most people just figured the Sha were somehow connected to an Old God that resides there and left it at that. I'm not saying that the Sha can't be connected to an Old God, but my personal theory is that the Sha are souls of dead Pandaren that respond to the emotions around them.
Think about it, Pandaria is a lovely place with people who seem deeply spiritual. What if part of its beauty is that the spirits of dead Pandaren of responding to the Pandaren's mostly positive outlook on life? Then the Sha come when bad emotions and hatred flow and make things more sinister and dangerous.
Time will tell if my theory is correct or not, but I would personally love it if it were true. I think it would not only add a darkness to Pandaria, but it would also add a darkness to the Pandaren, whom so many people complain about as not being serious. But this all brings me to my last point, which is really more of a desire/prediction.
Ghostbusters 2 had a river of ectoplasm that responded to human emotion. It also fed that emotion back into people that were near it, but let's ignore that part. In Ghostbusters 2, when things were at their most desperate, the Ghostbusters coated the inside of the statue of liberty with the ectoplasm and used strong positive emotions that were stirred by an inspirational song to animate the Statue of Liberty to penetrate the defenses of the museum.
What of the Sha could also possess objects, similar to a poltergeist? What if, in the Siege of Orgrimmar, Garrosh's defenses are too strong to break without losing too many people. To combat this the Horde and the Alliance bring the Sha to Azshara to infect the giant Gallywix face and then use strong positive emotions from their cooperation and an upbeat song to animate the giant face, bringing it out of the ground with a body to break the defenses of Orgrimmar?
I wanna do that raid fight.