Every nation on earth banding together and unanimously declaring war upon Equestria and Celestia, particularly if you've read other TCB stories...
Heck, many moments in this story are only as good as they are because it's a deconstruction of The Conversion Bureau, and we finally get to see consequences given to the ponies.
When the barrier finally goes down after sustained bombardment.
The Longest Day confirms that humanity reclaimed South Africa and are on their way to winning the war.
In Aftermath Part II, Celestia finally gets a What the Hell, Hero? lecture from Queen Molyneux of Gryphus, calling her out for her Assimilation Plot. What's more, it's made very clear that now that the gryphons — and soon, the rest of the world — know what she's capable of, her Omniscient Morality License is as good as gone and she will answer for her crimes.
For those who read so many other TCB fanfics where Celestia has the moral right, and no matter what the humans do, the ponies are superior, it was very, very, very satisfying to see Celestia run like hell from the forces of humanity united. Granted, some of us wanted to see Equestria smashed into a pancake, but it was still satisfying nonetheless.