This is, I believe, a systemic problem with the user base of Ask Different*. Sites like Stack Overflow, where the approach to Q&A was pioneered and little better ingrained in the user base, don't seem to have it quite as bad as we do**.
We attract a lot of search queries for problems and solutions that would otherwise have lead people to more traditional Q&A type stuff: MacWorld, blogs, traditional forums, etc. We have great content here that covers a wide range and great depth of subject matter, so we pull a good deal of search traffic from these topics. Unfortunately, the people looking for answers to these topics tend to not be greatly familiar with our Q&A approach: if you like something, vote it up. We get a lot of "thank you" type posts and comments we have to clean up as mods -- which is pretty clear indicator if you ask me that people visiting here often don't get that you don't write a thank you, you say thanks by clicking the upvote button.
So how do we solve this problem? That, I'm afraid, I have no good answers for.
* Completely speculative, I have no hard data for this.
** Also speculative, don't press me for data 'cause I'll crumble under scrutiny here.