So simple, yet so complex...
I think the main thing I look for in an instructor is solid self confidence, without even a trace of arrogance.
I want an instructor who knows how to teach a physical skillset, and that can be a lot to ask. Any number of people can teach an academic subject where all they have to do is stand and deliver information at the head of a classroom. On the flat range that won't work. A good instructor can demonstrate whatever skill is being taught, can do it left handed, right handed, with any make or model of firearm likely to show up on the line, and can show alternatives that will work for shooters whose handicaps prevent them from doing it the normal way.
A good instructor is totally observant always puts safety first.
A good instructor is a consummate diagnostician, who can see where a given student's problems arise, and can quickly offer multiple practical fixes.
If an instructor has a "my way or the highway" attitude, I'll take the highway.
A good instructor knows that the fundamentals come first, and that an expert is one who always performs the fundamentals well.
A good instructor has a sense of humor.
A good instructor understands pressure and its effects on students, and knows how to build pressure gradually on the flat range as students progress by adding ever more demanding drills.
I spent the biggest part of my professional career working in the schoolhouse where Special Forces soldiers are trained. The first thing that happened when I went to work there - at least it was way back when - was that you were sent to ITC, the Instructor Training Course. You got taught how to teach the Army way. "Tell 'em what you're going to tell 'em, tell 'em, then tell 'em what you told 'em." Every one of us had that in common no matter where we came from or what we were doing.
What I learned over the years as I worked there was what a deep mastery of both the skill at hand as well as the art of successfully teaching a practical skill could do. Imagine a successful Special Forces soldier, who is (well, was anyway) essentially nothing but a glorified schoolteacher. Imagine that soldier spending 20 years or so doing the job at increasingly higher rank with increasing responsibility. Then consider that soldier retiring and going back to work as a civilian instructor, teaching new generations of SF soldiers that skill - for another 20 years. That's the depth of mastery of both the subject and the art of teaching I got to witness first hand for over a decade. Not just in small arms but commo, engineering/demo, field medicine, intel, HALO, language and everything else.
NOT to say that it takes 40 years to get to be a decent instructor - but it DOES take time. And the most impressive thing I saw with SF soldiers was their ability to be either 100% teacher or 100% student, and to transition between the two at the drop of a hat as circumstances required. Any good instructor is first and foremost a perpetual student. No one ever knows it all, and as one of my personal favorite instructors says, "The state of the art is a moving target."
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.