jimbeetle - 9:21 pm on Jan 25, 2013 (gmt 0)
Well, you have more than one data point. such as these you sussed out:
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.
Now that's interesting, an obviously healthy site that is keeping its head traffic but is losing long tail traffic.
I couldn't hazard a guess as to what on-site factors would cause such a phenomenon. Have you happened to see any other reports of this happening?
I will hazard one SWAG, albeit in very small type: Is it possible that G's search suggestions are starting to funnel queries away from the site's historical long tails? But that should still be something seen by other folks.