Good instructors are also grounded and have perspective. Whether it's because they know they have above-average monkey skills, knowledge, and/or SA, or just because they tend to be of the laid-back persuasion, it's essential in my mind for an instructor to be comfortable with giving a stud rope to hang themselves with, while still trying to guide them back to the path as they start to go down the road of dorking something away. They will challenge above-average studs while knowing that they've moved into the realm beyond the minimum, where it may or may not matter if a stud gets something right. They will not make things personal when a stud screws up, and they know to recognize the big screwups from the little ones.
I think you usually see the converse of this when an instructor is bitter at his life, he's insecure about his own abilities, or he's excessively egotistical. Students generally think he's an ass, and don't want to fly with him. He may keep studs on such a short leash that they never learn anything,because he's not comfortable with his own ability to save the situation otherwise. The pissed-off-at-the-world type may belittle or humiliate them when they screw up, in excess of what may be necessary to "make it sting" for a lazy or unmotivated stud. It's one thing to deliberately jack up the stress level in a calculated manner in order to see if the stud can handle pressure. But if you're an LT and you have an Ensign standing at attention in front of your desk for anything other than the most cosmically negligent and stupid boneheaded move known to man, you're doing it wrong, and need to reexamine your temper and how it affects your teaching.
Other pitfalls I've seen are the instructor who has the NATOPS/FTI memorized, and consciously or subconsciously uses this as a tool to intimidate students in the brief. This also meshes somewhat with the guy who sets aside as much debrief time to talk about using the wrong color on a briefing board as he does to dorking away an intercept. Teach a kid to kill the closest alligator first. Sometimes someone who is insecure about his aviating abilities will show off his memorizing abilities to try to hide his flaws. Other times, huge brains just can't interact well socially, don't work on improving this, and don't realize that this cripples their ability to teach. The converse of the insecure types are those who have the best stick skills/SA out there, but are jaded and hold struggling studs in contempt, consciously or not. It doesn't matter worth a damn how good you are if you can't turn around someone who needs help and make them into your replacement if it's possible to do so.
Just some Saturday thoughts from someone who spent way too much time in the TRACOM.