nothing new... in the past the male hero of Diablo 1 turned the diablo of the next game, and the warcraft have a long story of do that.. the best example "Arthas". XD
Never noticed - although you could take Sylvanas out of the image... pretty much yeah. She's not that much of a bad guy, just pissed at the Scourge and humans for being f*cked over, like all the other Forsaken. Her intents are still somewhat... noble.
Anyhow, I think the gender issue is probably just random. Either that, or there's actually some hokey pokey culture things that make that sort of character more compelling.
Something that DOES sort of annoy me though with their recent work in WoW was the ending of Cataclysm... Dragons were supposed to be the ultimate guardians of the world. Now they're rendered obsolete. From a larger perspective, nothing matters anymore - story-wise at least. ...Still love MoP though. I just think they f*cked the lore with the ending of Cata.
Leah turns demonic and becomes Diablo.
Sylvanas turns undead becomes Forsaken.
Sarah Kerrigan turns Zerg and becomes the Queen of Blades.
What's with Blizzard and having all of their female protagonists switch sides and become bad guys?