These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moral Event Horizon: In-universe, Celestia believes she crossed this when she chained The Sun and forced Her to bend to her will in order to avoid setting at an extremely inconvenient time, then kept Her bound for over a thousand years.
Tear Jerker: The entirety of the final chapter. It will crush your soul to dust, then turn that dust into beautiful sculptures.
Make that the whole of the "Eternal" two-parter before the epilogue, thenthe Epilogue itself. It's a really long, but oh so beautfiul series of Tear Jerkers. Or, hell, make that the entire thing when taken as a whole. The whole fic is just...beautiful, especiallly when you've taken the time to see just how much Twilight, Celestia and Luna had to go through, to reach the conclusion at the end.