hitchhiker, I don't have an answer but perhaps another data point for you. I also run a Q&A type site (much smaller operation judging by your numbers) and it also lost traffic precipitously on Aug 1st. So, there may actually be something about the type of a site that is.
Many people here pointed out the issue with pages that contain a question but not an answer. I do not share netmeg's righteous disgust with such pages. For one thing, they DO have content. Some of the questions we get asked are so esoteric, it makes me wonder how the person expects any kind of answer at all. Even then, someone else looking for a similar issue may get clues from another person's question or at least know that he's not imagining things and someone else in the world is looking for a solution as well. Often times the two people looking for a solution get connected together on such page, even though it does not contain an outright answer.
Still, I have analyzed which of my pages are getting Google traffic (through serious data collecting and log processing, not anecdotally) and I can tell you that Google already has a very strong preference for pages that do contain answers. It may be preference for pages with more content in general or they may able to analyze the structure of the page and see/count the answers (they are formatted slightly differently from the question in my system, also custom-coded). Regardless of how they do it, a page with an answer is already 10+ times more likely to get a visit from a Google's SERP than one without an answer.
So, I am also considering no-indexing pages without answers despite my personal conviction that they do have some value. The only thing that's stopping me so far is that I don't want them to be buried forever in case they do get answers in the future. Does anyone have experience they can share with indexing pages that were previously no-indexed?
One more thing: a behemoth-sized competitor in the field is choke-full of no-answer question pages. They do make it look more like a search page, actually:" we don't have an answer but perhaps you'd be interested in these pages?" - the list follows. You open those and most don't have answers themselves and just send you down the list. I'm anxious to see their August traffic figures on Compete - would be interesting to see how they weathered whatever change we got caught in...