This has come up many many times in the past - http://www.linuxquestions.org/questi...hreads-159778/
for example, from 2004. The issue is that a thread needs to be marked as solved by the thread starter and most of them don't even return to say "thanks, suggestion number X solved it", so why anyone would think that they'd come by to click a button.....?
If you ask the mods to do it, well that's unworkable: firstly there are far too many threads and far too few people for the job and secondly, how will anyone other than the person involved know whether a thread is solved or not? Also, after solving one problem, posters often return with follow up questions that are only slightly related, when do we count a thread as solved?