I agree 100% with "namelessDancer". Expressions like "pick up your posture" may work for aome people, but if i can't get it reasonably quickly, i need a more in-depth explanation.
It's very difficult to define the qualities of a good instructor as pupils are all very different - learn in different ways, have different needs and so on. This sounds very vague i know, but one of the skills of a good instructor is to recognise, and satisfy, the needs of the pupil, and to adapt their approach accordingly. This is necessary in all branches of learning and education (i'm an educator myself - but not in dancing -- if only!!)
They say teachers are the worst people to teach!! and there's something in that. If my coach is reading this - you're doing a great job.