Great question!
Personally, I love honesty from my students. From needing a bathroom break, to being cold, scared, bored, confused, overwhelmed and so on. I can read body language, but often guess wrong as to the cause for emotions.
Stating a goal is helpful, but I realize many students don't know enough about skiing to be able to define it in words.
Being willing to accept that in a group lesson some goals may not be realistic due to varied abilities, conditions, limited time, and most importantly a few moves may need to be learned prior to embarking on chutes, big moguls, etc...
A willingness to drop down to an easy run for a hard or important exercise, assuming the goal is technique improvement. For some students it is not, and that is fine too!
Being understanding with instructors as we don't always have all the answers, but hopefully try our best to find them.
We know that some students need to understand movements from a more verbal and technical point of view, but in group lessons, we at times have to cut the more talkative students off in order to get in enough practice time. Asking questions on the lift is ideal.
Recognizing that chemistry between people is very important and that if the fit seems off it is not a personal matter. Many ski schools will try to accommodate students who prefer to not go back to an instructor that was not a good match.
Please remember that a student's safety often trumps the learning and the fun, and occasionally we may need to gently reprimand/remind students, especially younger ones, about what is going on.
My one pet peeve is when parents take lessons with their kids and then assume they are equally skilled at giving feedback and pointers. While keeping hands up may be something I stress on a wide green slope, it is really not that important when negotiating a steep and narrow and busy traverse, where not having the child get hit is my focus...but not always the parents'.
Finally, I get confused students at times who tell me their previous instructors told them the exact opposite of what I am teaching. For example in a recent women's camp I was encouraging a gal to use her poles and she told me her instructor in Europe this year told her poling is a waste of time and not to bother. Now, I suspect he was actually working on something with her, but that is what she got out of it. My point is to recognize that as our skiing evolves our new movement may become exaggerated and a return to the opposite might be necessary. An example would be working on a taller stance, where perhaps a student looses the ability to then get small or retract for a bump. Ask the instructor to clarify if things seem backwards...there is usually some wisdom in their apparent madness.
Going back to the beggining, feel free to tell what you enjoyed and did not enjoy in the lesson...it helps us become better instructors and coaches.