walkman - 3:09 am on Apr 22, 2011 (gmt 0)
I could see even a 20% factor in this, but they are many popular and successful sites that got screwed by Panda, very sticky ones. So I am guessing if your pages are "spam" you are toast. If they aren't, then these metrics may help you.
Then there's Matt Cutts not ranking for his own article