There are many reasons why people might dislike the trailer.
Sylvanas fatigue: She’s been in 3 expansion trailers in a row. It’s the same issue people have with Jaina and/or Saurfang being in every cinematic.
No foreshadowing in-game: Sylvanas’ power-up was never explained in-game, and the Jailer was only ever slightly hinted at during vol’jin’s quests which half the playerbase didn’t see. It was only until the very end of the war campaign that NPCs actually started commenting on her new power.
She is a reminder that Blizzard couldn’t commit to a faction war-based expansion without immediately resorting to dragging in cosmic horrors during the very first raid. Reminder: Only 1 out of 4 raids in BFA had anything to do with the faction war.
Insane power scaling: Saurfang, an old Orc managed to scratch her, yet the Lich King and his army of undead couldn’t even touch her. If this takes place directly after BFA then where did this power-up come from?
Direct continuation of the current expansion’s story. More than any other expansion. Makes the story feel like it should be part of a patch instead.
Yet another Horde leader does something bad. We’re running out of Horde racial leaders because Blizzard can’t resist hitting them with the villain bat. If it does turn out she is doing all this for the greater good, then prepare for the rage on the forums from the NE players who still haven’t gotten over the tree.
And many more.