From what I can see, the reasons are as varied as the players.
The edgier types feel she’ll dull the Forsaken’s “death to the living” edge, and humanize them. Some have an argument somewhat akin to cultural appropriation: she simply identfies as undead, and was given a blissful ressurection, instead of suffering the true agony of the Scourge’s necromancy. Some just hate that she’s a Menethil. Some hate her popping up and expecting loyalty after years of absence. Some think of her as an antithesis of the Forsaken; raised by the Light, when the Light used to be a bane to the Forsaken, and the pious among them were the Cult of the Forgotten Shadow. Some are suspicious she was raised by a Naaru specifically, after we’re being shown they’re not quite so benevolent.
Or any combination thereof. Or more.