What makes a good pitcher? That's a great question.
There are certain common traits that all great pitchers have. Number one, they all have a fluid delivery, with no tension in their bodies. Number two, they can throw strikes. A third facet that they all have, as a component of their pitching, is they do not do anything that's going to hinder their arm, meaning they're not going to throw in a manner that might injure them.
An excellent pitcher might also be able to keep hitters off balance by varying their timing. They might throw some four-seam fastballs, two-seam fastballs, be able to rotate over to a change-up grip, throw an effective slider or curve ball, and in doing so, that will keep the hitters guessing what's coming. Another thing pitchers might want to do is vary the eye level. So they might start with a pitch at the eyes and then change down to a pitch at the ankles.
All talented pitchers have one common trait. And that is, they can all throw a pitch from the same arm spot and the same arm speed whether it's a fast ball, change-up, curve ball, et cetera. So it's very important as a pitcher to be able to throw the same arm speed and arm slot every single pitch, regardless of what that pitch is. Lastly, all great pitchers are effective at holding runners on base, in doing so, the opponent cannot get a good jump on the bases, on them. If you can do all these things, you will be an effective pitcher.