There were some high tensions in Cataclysm between everyone's favorite Banshee Queen and Hellscream. However, his recent shift in what he deems 'honorable' might have grown to include the Valkyrie and plagues.
I kept waiting and waiting for her to be tossed in the mix and see which way she sides. Honestly I don't think the writers knew what to do with her. With all that at's stake and Sylvanas' personality I doubt she would just sit it out. I thought we would here from her around the time the escalation with the Blood Elves occurred.
So do you think that her absence is just lazy writing? It's been my major lore qualm this expansion.