I agree. I've thanked someone and been downvoted before. When I asked what was wrong with thanking people, I got down voted even more. The end result is that I don't thank people any more and I now firmlly believe we need to reform the XP system to include more accountability in it.
When I comment on someone else's problem, I'd like to know if my answer was helpful to them. When I'm searching to find a solution to my problem and I find a node that has a number of different answers, I'd like to know which one worked best or was chosen as the final solution. A quick thank you reply to the best answer is perfect for this.