Joel being killed off when he was half the heart and soul of this series. The other half being Ellie.
I seriously think a better direction for this series would have been Abby seeking vengeance and Joel/Ellie having to fight her and her cohorts: another go for two longtime fan favorites. Abby's existence would have presented an inconvenience for Joel and his lie as well.
Something like that would have been better, yes. I actually expected Joel to die, but not like this. The problem is that the story from Abby's side is not set up properly and her as a character isn't built up with sufficient time in order for us to care for her even one bit. She just shows up like this roid raged monster that brutalizes Joel and we're supposed to appreciate "her side of the story".