If class was good, tell your instructor. It just might make their day.
Some days everyone is quiet at the end of class. As a yoga instructor I always wonder is that a good quiet? (like I am just all chill right now) or a bad quiet? (like you just killed me or that class totally stunk). I never know and always wonder. Sometimes I will get lucky and someone will stop me later and tell me it was a great class. Or that they loved class. Or even that they feel better after it. Or they apologize for not having kept up with me. Which is never needed as I consider someone who is listening to their body and doing what they need to do, a great student and, one who understands I am only a guide.
I will take any comments, even criticism (although the highly sensitive person in me is still learning to let the harshest ones roll off my back), because it gives me feedback I can work with. Usually there is always someone who doesn't like class --and that is okay, too. Not liking a class can be because it was too hard/too easy, it worked a body part that was weak in the student, or it didn't work the student as much as they wanted. Sometimes a bad class is nothing at all to do with the instructor, it can be the student having an off day or week. They get too hot, or are dizzy, weak or off balance or they are just plain too tired to do things the way they normally do.
What your yoga instructor wants to hear is truth.
Tell them when you feel great after class.
Tell them when you feel sore the next day.
Tell them you loved the song they played in shavasana.
Tell them you were finally able to connect your hands in the side angle bind.
Tell them yoga is the highlight of their week.
Whatever it is, please feel free to talk to them. We are human and we want to know how we are doing, how we can be better, or just to hear that we are making a difference in your life/body.
Never force it. Never lie. Just share truth when you can.