Frankly, you can do only one of the two: A MMO story, or a complex one.
Trying to do both leads us to where we are now: To understand why anything is going on, read a stack of novels, because it's too complex to be shown in-game.
Note that "Not complex" does *not* mean "Uneventful", showing action in a game is easy, what i mean to say is that complicated schemes and hidden motivations (Especially the latter hurt Sylvanas' in-game storyline) are very difficult to show in a game (Even in cinematics) in a way that doesn't feel forced.