However, Google seems to be on the verge of removing that feature.
http://productforums.google.com/forum/#!topic/websearch/Psb6OmlLJTg[1-25-false]
It is no longer in the dropdown and to use it, you need to add &tbm=dsc. How long this will last is anyone's clue, but with it being removed from the dropdown list, I expect the filter to get worse over time.
So, does anyone know of a search engine that allows for searching similar to Google's Discussion filter? A search engine that focuses on looking for information in forums, comment threads, and places like StackExchange?