Random story line shifts...
for example:
Air Gear: Chapters 1-160 roughly is a developing relationship between the main female and male characters, then they add a new female and write off the original..... out of the blue....
This pissed me off so much it was rediculous ( they have left a few area's open as to who he would end up with but since around chapter 160 it seems to completely lean toward the newer girl, including the main char's personal feelings. )
I really fall in love with a few characters that get developed through storylines. One of those would be Ringo from Air Gear, when a character like that is screwed over in such a random out of place manner it gets on my nervs ALOT.
( Mind you something like FMA where they kill off Hughes I can understand, He was my favorite character... and yes I was sad to see him go but the impact served a purpose. )