In my honest opinion, there are two key things that make someone a good umpire. Those two things are consistency and positioning.
Consistency usually applies to the plate umpire. Every umpire has his own zone when behind the plate. Yes, there’s a predetermined strike zone according to the rule book, but the reality is everyone sees that zone from a different point of view. Some umpires will give a little extra around the knees, but you’ll be hard pressed to get a call at the top of the zone. Some umpires, like myself, will give a little on the outside corner, but will call the inside corner very tight since that’s the easier of the corners to see while in the slot position. These variations in the zone don’t matter as long as it stays consistent throughout the game. When a batter steps to the plate for his second or third at-bat, he should be able to distinguish the umpires zone and know what to look for.
Positioning is key when it comes to seeing a play, or at least convincing everyone that you saw the play. As mentioned above, the slot position being the plate is the ideal place for an umpire to set up. It allows him to see both the inside and outside corners of the plate, and gives him optimal protection from foul balls that go straight back. The slot is the areas between the catcher and batter on the inside corner.
The other part of positioning is for calls on the bases as well as plays at the plate. Positioning is important for force plays, but it’s even more magnified on tag plays.
For tag plays, the umpire wants to get into a position called the wedge. This is the area off the base side hip of the defensive player. As the play develops, the umpire should rotate with the moving of the players glove as he applies the tag, becoming an extension of the arm from the backside of the defensive player. There is also the front side of the wedge, but most umpires try to avoid that position unless it’s absolutely necessary.
If an umpire can maintain consistency and good positioning throughout the game, he should be able to keep the coaches and fans out of his ear for the majority of the game. You’ll rarely see a manager argue ball and strikes when they’re being called the same both ways, or safe and out calls when the umpire is right on top of the play.