My single umpire complaint has always been the arrogant notion of personal strikes zones... i.e. "my zone", as if the rules allow umpires to personalize the strike zone according to individual taste. While this obviously happens all the time, it is also a disservice (to both teams) when pitchers or hitters receive a strike zone advantage.
I have no beef with the fact that 15% of ball/strike calls can go either way but, for example, when an ump clearly rewards a "command" pitcher who can consistently hit a spot thats a full fist off the plate..... he gives an advantage to one team and eliminates any possibility of fair play.