When Princess Celestia looks into this world's version of Truth (here just a single drop of water), she sees a myriad of worlds, some of which are quite awesome.
In one world, Destruction managed to fight off Discord when he tried to eat him, and went on a mad rampage, until his 3 siblings stepped in to stop him.
In another world, Pandora makes a Heel Face Turn and becomes Equestria's protector, disguising herself as Cadance and taking her place when she dies fighting Havoc's avatar.
Perhaps the best one though is a world where Morning Star, Celestia and Luna's fallen brother, invaded Equestria, and came within a few moments of destroying all the heroes. Right before their deaths, Twilight declares that his claims of perfection are meaningless while she has her friends. And joining in a power hug, they call down the Fires of Friendship (the power of empathy personified and an Elder Alicorn), to defeat Morning Star. "Both (Celestia and Luna) were never more proud of the six friends standing before them."
Pip acts as a Spanner in the Works so that Nightmare Phobia's plans to completely alienate Luna from Ponyville failed.
The fact that this story exists, and provides a credible way to redeem a version of Discord by showing the worst he could be.
Discord going Papa Wolf to protect Fluttercruel from Nightmare Phobia.
Discord giving Nightmare Phobia the And This Is for... treatment for killing the constructs of Rarity, Rainbow Dash, and Applejack. For the first he hits her with a giant sword of magma, for the second he smacks her with the construct world's miniature moon, and for the last he hits her with the miniature sun.
Discord and Fluttercruel tearing Nightmare Phobia's pre-One-Winged Angel form apart from the inside when she tries absorbing them.
Fluttercruel managing to drive back the darkness of Nightmare Phobia's One-Winged Angel form, saving Discord and the Princesses and containing Phobia long enough for Discord and the Princesses to use the Elements of Harmony to obliterate Phobia for good.
Speaking of which, Discord somehow managing to use the Elements of Loyalty and Laughter in concert with Celestia and Luna using the other Elements to tap into the harmony of every living thing in Equestria to destroy Nightmare Phobia for good.