So if I may start (not listed in order of importance, just what comes into mind):
A great trainer:
- explains what to do, how to do it and WHY that is done (not just do this, do that, but you do this like that, because it has this effect on the horse, you and what you want to do)
-answers all your questions to the best of their knowledge and is happy to answer questions training related or just horse related in general
-cares for horse and rider
-is honest
-teach towards your own horse, I.e. Teaches you things you will be able to use inside and outside of the ring (example: my trainer is teaching me how to lunge and round pen - I have NEVER had anybody before that teach me how to do it right before and I love it because reading on this forum is many problems are solved on the ground, so once I have my own horse I will be able to apply those skills)
-can give you valid reasons for his/her training method (i.e. I use a lunge whip instead of just twirling the lunge line, because xyz, or vice versa)
-is knowledgeable not just about the riding style, but also horses in general (health, behavior, etc)
One I think of more I will post. But you all feel free to add and post your own views.