@jimbeetle: Thank you for your input, Jim. Yes, of course I am looking for all possible factors but the idea of inferring some data from the Panda updates not having an effect would be to prioritize my actions. For example, if I knew that this means the content is considered a-OK, I would concentrate on getting good links instead of writing new content. Not too many links were gotten since 2011 and natural link accretion is low due to low overall traffic.
Does this mean that I can skip looking at the user experience factors and concentrate on adding features instead (for example, site search may need some work as it's getting antiquated)? And so on. I do realize that ultimately all of that needs to get done but if I can prioritize the tasks, I can do more good in less time, hence this question: I have this one data point, what can I do with it?
As far as the factors that I can see leading to the loss of most traffic: it's definitely the hollowing-out of the long tail. All the top KWs are still there, but tens of thousands of those that used to bring 10-20 visits a month or even less are no longer there. Likewise, all the top traffic pages are still top pages (and number of visits is roughly the same) but those pages that were visited just a few times a month now get no traffic at all. Given that there were thousands of those, the effect on overall traffic amount is considerable.