Reasons I chose (and kept) my current instructor:
-- She's nice without be a doormat. She's also laid back but not careless.
-- She's younger (25) but very experienced
-- She showed IHSA in the same region/zone as I did. So, I was familiar with her instructor and team, and she was familiar with my old coach and team.
-- She pushes me when I need it, but knows when to leave well enough alone when I'm just having an off day.
-- She runs her barn the same way I would run my own. She's practical, emphasizes safe riding, and refuses to progress students until they have the basics down.
-- We also share similar philosophies about riding, horses, training, etc.
My old instructor wasn't really a choice, as he was the head trainer at my college, when I started taking a college riding class. But he is now the gold standard for me as far as trainers are concerned. He was also a somewhat BNT -- something I didn't know until I left college.
He was gentle, but pushed us when necessary. He was fantastic about coming up with ways to tell us what was wrong and how to fix it. Phrasing it in ways that made it click in our head. He was also incredibly warm, had all sorts of funny stories/anecdotes for us while we were riding and never, ever made us feel stupid or inadequate.
And even though I only did walk-trot classes in IHSA, he was ALWAYS there before we rode into the ring. He would talk to us, adjust our stirrups, take out our legs and roll them back, wipe off our boots and give us words of encouragement. Even though he had 25 riders to look after, from sun up until just about sun down.
He wasn't in it for the ribbons or recognition -- he was in it for us. And I miss him dearly.