Most of the fans had prior experience with the franchise
The movie does not explain at all who sargeras is and he basically shows up randomly. Critics expect a high fantasy plot but it's basically a monster movie in a high fantasy setting with this setup.
I liked it. But it's not what people expect from high fantasy
Reminds me of this article: https://www.kcrw.com/news/shows/press-play-with-madeleine-brand/vaccines-homestead-film-reviews-appetizers/melody-gutierrez-wonder-valley