On the one hand, I'd like to thank people who provide useful answers, but on the other, I can see how those comments really just become clutter for everyone else.
The founders of the Stack Exchange network have written about this exact topic here.
It takes a bit of getting used to but thank you, as well as greetings and signatures, really is just clutter. Of course it might have been different if we didn't have the voting system.
On a personal note, if you ever do feel compelled to post a
thank you, please, please, please don't abbreviate it to
thx or some other absurdity. I shudder ever time I see and makes me thing there are too many 13 year olds on the site.