Winter War/YMMV

Complete Monster: Aizen, of course, arguably even moreso than in canon. After all, he Would Hurt a Child before experimenting , is letting the worlds literally fall apart by leaving Gin in charge of Seireitei, and stuck the captive members of the Fourth Division with remote-detonating collars. Oh, and he employs two minions determined to be the more depraved Mad Scientist... and Gin. Who also Would Hurt a Child, among other unplesant things.
Crowning Moment of Awesome: Orihime rescues Ichigo. To be precise, she rejects his hollow's control of him, restoring him to control of his body and saving the lives of most of the people in the vicinity. After being held captive in Hueco Mundo for several months.
- Isane being the one who realizes how to defeat the Barragan fragment. She's been through so much in the fic, and then after just losing her hand, she's still thinking fast enough to come up with a way to kill it. (Everyone else was pretty awesome too, but her Failure Knight condition at the beginning of the story makes her effectiveness and quick thinking here even more cool.)
Fridge Brilliance: Ogidou's behavior when he finds out that Hisagi killed Iemura- he tries to kill him during a battle, when his back is turned. How he reacts just seems to be an example of him going (more) Ax Crazy... Then you remember that in canon (Soul Society-arc omakes- these are very minor characters in canon!), Ogidou is in Iemura's squad of Fourth Division. They worked together day-to-day, and Iemura was Ogidou's immediate superior officer. It makes him trying to murder Hisagi seem less like pure Ax Crazy, more revenge.
Fridge Horror: Remember Kazeshini hesitating so long before agreeing to Nemu's I Cannot Self-Terminate? Perfectly understandable reaction... And then you get to "Crossfire." Tatsuki is killed by a clone!Nemu right before they disappear... meaning that if Kazeshini hadn't been quite so reluctant, she might've survived.
Foe Yay: Whatever thing Mayuri has for his former boss, it seems to be mutual and is definitely strong enough to suppress voice of reason in both of them. The same goes for Mayuri and Szayel Apollo.
Jerkass Woobie: Kazeshini, once you get to his POV.
Moral Event Horizon: If Mayuri hadn’t already crossed it, severing Nemu from her zanpakuto would definitely count. It was enough for Grimmjow to decide he's too disgusting to fight.
Nightmare Fuel: Hollowfied Shinigami, re-opened Hollow Pits, most of the characters' states of mind, the state of the universe in general, and on top of it all it somehow keeps getting worse.
- The Barragan fragment's corrosive blood. Meaning, if you actually manage to hurt it, you might just loose a limb to blood splatter. Or your zanpakutou.
- Chapter 43 revealing that while Kyoraku has been imprisoned and not turned into a Hollow, he has been subjected to Aizen's illusions on what appears to be a regular basis. This gives a Mood Whiplash moment of the group finding him apparently asleep in his cell, snoring no less, and Lisa getting his attention by kicking him. Kyoraku knocks her to the ground, puts his arm around her neck, and calmly asks why 'Sosuke' thinks his tricks will work this time.
Ships That Pass in the Night: Kon/Mashiro. In the fic, it makes sense- he's been using Ichigo's body full-time for months, she's been living at Urahara Shoten so that Urahara can help with her... condition, and he visits frequently. In canon, have they even met?
Tear Jerker: And holy freaking HOW!!!
- Well, here we have one war lost with great casualties and the other war ongoing. And authors put that 'very dark' warning into each chapter summary for the reason. And almost every chapter tries its best to break your heart at least once.