There are several WoTLK Classic Gold reasons why it's been controversial. First of all, Sylvanas in the past few expansions has committed a number of quite heinous crimes in her quest to collect souls for the Jailer, including , but not only:

The Alliance is waging a full-blown war against the AllianceBurning down a massive tree Teldrassil which is the reason the Night Elves called homeKilling Horde leader Varok Saurfang, who was trying to remove Sylvanas as the head in the HordeKidnapping and making leaders suffer torture by the Horde and Alliance

Many players, specifically Alliance and Night Elf players, have been looking for revenge against Sylvanas. The film's plot does seem to imply Sylvanas's former Banshee Queen may need to pay for her war crimes in the future however, the fact that Sylvanas has reclaimed her soul and fought back against the Jailer all point towards an ending that many fans weren't keen on viewing.

It could be months buy WoTLK Gold before fans learn what's coming next for the WoW storyline once the inevitable patch 9.2 is released, but just when players thought Sylvanas would finally reach her end, they discover she still has a big role to play, either regardless of the outcome.