my_name - 6:35 pm on Nov 10, 2013 (gmt 0)
I was also hit on September 4th with a 75% drop in traffic. The web site has never had a purchased link and never had any blackhat SEO.
On the day of the drop I had 240 crawl errors (I didn't think much of them as they are legacy pages). I currently have 8,000 (because I removed a lot of pages that I thought Google could be seeing as spam, but now in hindsight they were unique original pages, but I'm sticking to what the web site does best now. These are returning the 410 gone header.
I have tried everything since to try and get some traffic back, but it hasn't happened. I have already started to work though the HTML improvements in WMT and they are gradually decreasing. But the crawl errors is something I have overlooked. I have started working on them. I have started on the 301 redirects from the legacy pages to the new urls where the equivalent new page exists.
Maybe the original 240 crawl errors caused the penalty...
Thanks for the advice. I will report back also. Fingers crossed this is it!