The screenshot of the impressions and clicks you posted is definitely very interesting, with the "off the cliff" type of drop. I am wondering if you could provide a bit more details, specifically what happened with your rankings since 27th.
I am asking this because there have been some reports of the traffic drop with ranking unchanged in this thread: [webmasterworld.com...]
I am wondering whether you could go to WMT Queries section and select four days from 27th Nov to 30 Nov, and below the chart select option "With Change" and then tell us what happened to your rankings - are they roughly the same or did they drop?
Another piece of information that would be very useful is the number of queries - to see if you have lost "long tail". So it would be good if you could check the number of queries WMT reports in the period 27-30 of November comparing to the period 23-26 November and post this detail.