When I leave feedback, I'm not necessarily looking for an explicit "Thank you," or even a response at all, but in general I do like to engage in a discussion with the writer. I tend to see through "Thank you" and pay more attention to any comments my feedback prompts. When people leave feedback for my stuff, I don't always remember to thank them explicitly, but I do respond a lot of the time. Unless I'm responding with snark or getting all defensive, I think that "Thank you" is implied.
"Too often we enjoy the comfort of opinion without the discomfort of thought." - John F. Kennedy on reality
"What if everything you see is more than what you see--the person next to you is a warrior and the space that appears empty is a secret door to another world? What if something appears that shouldn't? You either dismiss it, or you accept that there is much more to the world than you think. Perhaps it really is a doorway, and if you choose to go inside, you'll find many unexpected things." - Shigeru Miyamoto on secret doors to another world