Or maybe it's just that she's a compelling character that isn't just another trope?
Exactly! I think too many people underestimate this aspect. So few of the characters in WoW have any depth, so when we have characters like Sylvanus or Garrosh, it should be obvious why people like them. Sylvanus is especially interesting because her motivations aren't "LOLOLOL! I R EVIL RAWR!" or "I am the super-good-guy who always does everything right!"
I think the game needs more characters like that.