The bad ones are easy to remember. The good ones require effort to recall. If I have forgotten an instructor completely, he must have done his job supremely well.............I keed!
Two marks of a bad instructor,
1) Stereotyping and generalizing. "You're one of those guys who always likes to ____, aren't you." It degrades every situation, adds nothing to the experience, and creates enemies rather than allies. I've heard this, I've heard of others who heard this, and I've heard others complain about this.
2) A negative attitude filter that inflates every deviation and every wrong answer just to drive a negative agenda on the grade sheet. The tape shows +/- 50 feet and the IP says you were a hundred feet low all day. If there was ever a trend, it's the negative-attitude IP who makes a trend out of fabricating negative trends. 30deg AOB becomes 35, 800vsi becomes 1000, abeam becomes sucked, late becomes gay-late. Enough with the doom and gloom, guys. Our guys are typically pretty smart and there's no award for blowing the whistle the most.
Two marks of a good instructor,
1) Thinking independently. How many IPs do you know that evaluate a student based on what other instructors have told them about a student? It's a double-edged sword that helps and hurts, but whatever it is, it's rampant. The independent thinkers rise above the mayhem.
2) Putting the experience into context. The IP who can bring the big picture into the lesson is the guy who makes long-term contributions to your success. Part-task training is the way everything is constructed, and students are quickly buried in beating down the next closest alligator without regard for what it all means. This gets into my biggest tactical (classified) criticisms of the single-seat business today, but the IP that can provide the picture beyond the part-task is the one to fly with.