Plotlines and fights tend to drag out due to a lack of narrative focus.

Ichigo's Bankai being completely useless. Name one enemy besides Byakuya who was noticeably slower than Ichigo.

THE GOOD GUYS NEVER DIE (but the bad guys usually do).

Currently, all the big enemies are so powerful they have to be handed Idiot Balls so the good guys can win.

Characters keep getting Demoted To Extra or completely becoming irrelevant to the plot (Ichigo and Uryuu's dad, I'm looking at you). Plus the timelines don't seem to add up given what we've learned through the various arcs.

Another troper just popping in to say CHAD. He is the embodiment of this. I thought he was going placing and then he gets Put On A Bus. It really bothers me that we were given characters and started to really like them only to have them whisked away never to be seen out of flashbacks and fillers.

Renji never wins, and the poor guy works so hard. He did kill that arrancar, but that was a surprise attack so...

Started out fun an fast paced, devolved in the Dragon Ball Z levels of slow as molasses story arcs and battles.

Yammy = Espada 0

The fact that every time a big fight is established, we immediately cut to a less important fight until it's finished, meaning we have to wait a month or two for the big important fight. This happens with just about every fight too.

When a character says, "It's over," during combat, it's an instant indicator that this fight's gonna go on for a while. Ichego vs Aizen withstanding.

The creator keeps baiting us with "Aizen's death", only to show that the attack was completely ineffective, not damaging at all; examples include Gin's attempt on Aizen, Hitsugaya's stabbing

All the arcs that turn out to be entirely pointless and rely on everyone being an idiot. For instance, Orihime Rescue/Hueco Mundo could have been avoided if Urahara had just used his freaking brain, and the Espada 1-3 fights were just a waste of time since Aizen wiped the survivors out for not winning fast enough.

Aizen not bothering to use his shikai on Ichigo. Seriously, all he needs to do is release it, Ichigo gets hypnotised, Aizen wins. He's supposed to be intelligent

Writer fail + apparent lack of focus = a FILLER ARC in the MIDDLE of a would-be epic final-fight between Ichigo and Ulquiorra.

Rukia started out as the female lead and now it's practically a cameo if she appears. Ichigo not being seen for a year despite being the freaking protagonist isn't much better.

Every arc is just the Soul Society Arc with new faces. There is always some stoic character (Byakuya, Ulquiorra, Yukio), a character who just wants to fight Ichigo and eventually turns traitor (Renji, Grimmjow, Ginjo) and one of his friends ends up needing to be saved (take your pick)

Manga being more serious than the anime due to half the comical moments being omakes. Also, that damn shiptease for Ulquihime despite it never going anywhere the results never shown and possibly never will.