I can't see why people hated this. The whole concept of mirror world is a fan favorite, and arguably pulled Discovery out of the ash heap. Plus, there's an origin story, along with a really cool map that details the whole Alpha Quadrant. And, there's a fledgling Rachel Garrett to boot, for fans of TNG. There's reference to a bigger plot to keep the series moving for next few weeks (as I hope they made a few episodes). I would give this a better grade than TNG season1/1, which introduces a wimpy and aloof Captain Picard (whose only comparison was Captain Kirk).

The only drawback in my mind was sealing off the mirror universe with that bomb at the end.

Long live the empire!!!