It looks like Google has removed the discussions search filter for some browsers (currently using Chrome Version 32.0.1700.76 m).
Hoping this is a bug and not another part of the “simplicity” trend they’re on at the moment.
Solution #1: Append &tbm paramter to url
A temporary fix is to add “&tbm=dsc” to the end of the search url. SilentEcho has added this script to automate it if you’re going to be using it a fair bit: http://userscripts.org/scripts/reviews/293082
Solution #2: Change Use Agent String
You can also use a user agent string of a browser that it still works for (I’ve heard Opera but haven’t verified).
Solution #3: Sign Out of Google Account
This didn’t work for me, however quite a few people are saying it does the job for them.
Thanks to this thread for the links/info: http://productforums.google.com/forum/#!msg/websearch/Psb6OmlLJTg/NoW2hr-qytcJ