So far we've encountered one, who said he went away because he felt like Sylvanas just wasn't herself anymore and he didn't want to stay anymore ("I once worked for the so-called "Banshee-Queen", Sylvanas, as an apothecary. I used to admire her, but something's changed within her. She's not herself anymore. ") . He joined the Argent Crusade and is now at the Plaguewood Tower. Is name is Argent Apothecary Judkins. He actually says he broke his vows with the Forsaken for it.
We also have other former Forsaken living in other places, for example in Dalaran (Alchemy trainer Deucus Valdera), but they don't say why exactly they left.
Now for the first time we see Forsaken wanting something else than Sylvanas, more than just one at a time, for a bigger chunk of the Forsaken society and doing something about it. By that I mean forming the council, not deserting, because that wasn't the purpose of the council originally.