This is a very interesting question and it has received some very good answers. Therefore, I intend to provide my personal experience and draw a conclusion from it.
Theory of Complex Functions was one of the courses that was dreaded by most of students in our department during my undergraduate studies. It was not because the course was too difficult (though it was not any piece of cake) but because it was taught by one of the most stern professors of the department. The course was one of the most unpopular ones but when I took it, I found that not only I enjoyed the course very much, I found the teaching method of the professor and also his manners and the meticulous attentions that he gave to mathematical concepts of the course very constructive for me. The professor's teaching method was unorthodox in the context of our department but there was nothing inherently wrong or unreasonable about it. For the course I received a very good score and when the semester finished (and I graduated), I went to his office. When I entered, he gave me the same stern and distant look as always. I briefly explained to him why I enjoyed his course and teaching method and then sincerely thanked him for it. The reaction of the professor was very interesting. He smiled genuinely and became warm and friendly, thanked me for my gratitude, asked a few friendly questions about me, my career and future plans and gave a few advice regarding them. At the end of our session, he shook my hand warmly and walked with me to his office door.
In general, I believe that professors are humans and like any human being, they enjoy and appreciate any form of sincere gratitude. This is something that good professors need and deserve so that they can know that their efforts are noticed and valued by their students.