Hehehe, so we have excuses for none of the Alliance to show up, but one missing Horde leader is a problem?
I jest.
They mentioned Sylvanas in one of the early Q&As being involved in the events of the expansion, but I'm guessing they ran out of time. I agree I'd like to see all the races from both sides represented.
All in all, though, I'm sure it will be Orcs vs Humans & Trolls with token NPCs from other races at best.
I can't fathom whatsoever how the night elves can't be bothered to take action at all. I think hitandruntactic is most likely right. The Alliance movements and such will be referenced in the Vol'jin novel, the NEs likely waging full scale war against Garrosh forces in Ashenvale, drawing more away from Orgrimmar.
We'll just have to see with Sylvanas, but I wouldn't be surprised if they don't have her show up. Not sure the Horde before/after Garrosh would particularly care for her method of defying him with "Oh use the plague anyway, we'll be the Scourge after all" though.