The Saiyans becoming wildly OP and making everybody else irrelevant.
This. DBZ had a bad habit of wasting a lot of characters in general.
That colorful cast
More specifially to Z, too many characters didn't get to shine enough. In a series like Yu Yu Hakusho, they only had a squad of 4 dudes, but all of them got down. Kuwabara
In One Piece, all of the characters get classic fights. Zoro will go full
on opponents. Nico Robin will snap people's bones on some
type stuff. Usopp is basically the guy that starts off useless, but steps up and comes through at the very end.
I feel like DBZ wastes a lot of potential-- Gohan was done dirty, Piccolo was relegated to babysitter status post-Cell, and the humans like Krillin and Tien got left in the dust. Yamcha never even had a chance. ...and dare I say it?
Even the prince of all saiyans deserved more than he got. Goku and Trunks both got to demolish Frieza, Vegeta's turn should have been next (and I don't even mess with Vegeta like that post-Frieza saga).