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.
Crazy Awesome: Mashiro kicks asses while in Hollow-mode- which she can't control and is barely conscious and really violent during it.
On the villainous side, Gin, who is a vicious fighter, a clever tactician and able to continue fending off several opponents while dying. Also a trigger-happy petty tyrant and almost certainly clinically insane.
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.)
Orihime pulling off her plan to reject Kyouka Suigetsu. Thereby destroying Aizen's most dangerous power mid-battle.
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.
That said, in canon Iemura and Ogido can't stand one another.
Hollow!Kira has a huge sweet tooth and is constantly making and eating snacks. When the Hueco Mundo insurgence team makes it to the cells Hollowfied shinigami were kept in, one of them is littered with treats.
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.
If Mayuri's highly disturbing internal dialogue about how excited he is at the prospect of capturing Urahara are any indication, it's almost a certainty.
"Why, if he hadn't excised certain organs as unnecessary, they would have been positively twitching at the very idea."
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.
Paranoia Fuel: Later in the fic, the Hueco Mundo team goes deeper and deeper into Las Noches and finds numerous signs of Aizen's... activities, but no sign of Aizen himself. All while knowing he could watch them and mess with their perceptions from the very beginning - or, even worse, start doing that anytime.
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: 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.