The problem isn't a redemption story in general; the problem is that almost two decades of lore are taken by surprise through a convient hindsight villain and it leaves an unsatisfying resolution to everything Sylvanas' character has been up to in the meantime. Her major motivations, as known to us out of character, have been changed at least three to four times. This incosistency, the dubious and vague explanation provided by the latest cinematic and the various tragedies that preceded what we're seeing now - who haven't gotten a satisfactory resolution at all - leads to so many being disappointed. Most knew where the story would be taken by Danuser though; the Sylvanas "redemption&attonement" story was crystal clear since the day he released "Dark Mirror" that provided insight into Nathanos' background.