Today, upvoting is anonymous; the recipient knows they were upvoted, but they don't know who upvoted them, so it's not a public personal gesture. The only way to let someone know who likes your answer is to comment, often by saying "Thanks!" which adds nothing to the discussion.
There should be some non-anonymous thing I can do to communicate that I (personally) appreciate your response.
For example, Quora has an explicit “Thanks” button. Here's an example: http://www.quora.com/How-does-Twitter-make-money As you can see, it has both upvoting and a "Thanks" button. It works great; check it out.
I think we’ll see a lot more people posting great answers when they get positive feedback from a personal “Thank You.” It feels really good to receive personal thanks, especially from an expert who you respect.
Jeff says that the way to thank users is to upvote and "pay it forward" by answering other questions. I think that this argument goes against human nature, because the recipient doesn't know who did it (or even what they did). The point of saying "thank you" is to make a personal (ideally public) gesture of gratitude, which feels good to the recipient.