there are no messages in Google WMT about unnatural links / artificial links
Those messages were not part of Panda. If this is Panda, it is targeted at "shallow" content but how exactly is still not clear. Ideas include some combination of text analysis, user data, authority, trust, usability.
One thing seems sure - the Panda algorithm is complex and not all affected websites show the same pattern.
I think I see one of those patterns where some Pandalyzed sites seem to be hurt by a combination of scrapers and weak authority. In one case, we got the rankings back simply by asserting ownership of new articles through authorship mark-up, fat pings via pubsubhubbub and a 1 hours delay on the RSS feed. We did nothing else before the rankings return.
Point of information, Google ran the "Scraper Update" just before the first Panda - just to zero in on original authorship. Even at the time, it didn't seem to me that they had it nailed down very well.