Ass Pull: The original Chapter 8's reveal that Celestia and Luna were in reality Tirek and Grogar in disguise. While there was some foreshadowing such as Megan's description of a woman her father disappeared with, the prophecies of Megan's "second coming", and, most importantly, the prophecy made by the G1 Twilight, many fans considered it inadequate.
Broken Base: Over the now-retconned Chapter 8. Some readers were thrilled. Most considered the story to be ruined forever.
Canon Defilement: Chapter Eight's reveal that the Royal Sisters were in reality two of ponykind's most ancient and monstrous enemies and that everything they ever did to protect Equestrian was to set up their vengeance against ponies and humans was... not well received, let's just say.
Diabolus ex Machina: The now retconned Chapter Eight's revelation that Celestia and Luna are really Tirek and Grogar, and that their benevolent reign was all to get the Rainbow Of Light from Megan eventually, spending a whopping 5000 years on this plan.