Two things destroy good instruction when it comes to firearms instruction. The first is poor cocky attitude from an instructor and the second is lack of real world experience.
An instructor who lacks humility does not do a good job. And the instructor who comes straight out of the military probably lacks the interpersonal skills needed to work with the public.
When one is working with an instructor it's generally more benificial to find an instructor who has been a cop or federal agent because they've worked in and with the public. They've also got a better knowledge of how to survive on the street without a full tactical set-up and massive amounts of ammo and a large rifle. There's just not a daily need for this in the everyday life of most Americans. Most Americans need solutions that are low key and lightweight enough to be comfortable for everyday life while at work, the grocery store, or the movie theater and since law enforcement work in all of these places they have a better lay of the land.
The next thing is finding an instructor who is friendly and confidence building. If you can't ask them a question without them blowing their lid what use are they. You want someone who's comfortable to approach and keeps class sizes to a number where you get one on one time to really dial in your skills.