TBH, I think most of us respond to offer our support/thoughts, sometimes these responses strike a chord and get picked up for further discussion, sometimes they don't.
It would make a very dull thread if everyone went around saying,
by Mrs X, please can you help?
By Mrs Y, yes, do this.
by Mrs W, not really sorry.
By Mrs X, thanks mrs Y, that is really helpful.
By Mrs Y, thanks Mrs W, great of you to respond.
and so on, ad infinitum.
Bearing in mind a thread can go up at something like 300 posts an hour it would mean the op couldn't actually think about any of the responses and the thread would make no sense to late comers.
Personally, I think the MN way is to thank sometimes, but most of the time you just accept that your post has been appreciated and do not expect acknowledgement for it. It just seems a little needy to want to be acknowledged all the time!