I've considered a few things: content, reciprocal linking, links not on topic etc. I'm going to try and think through more on my sites that lost rank and those that didn't. I had one that went from PR3 to PR2 and a 6 week old one that went to PR4 (it has less links and duplicate content so no answers there yet). I have noticed that in a couple of places I seem to have held PR where other pages didn't that appears to be outbound links helping. Some-one commented earlier that they thought Google was devaluing the benefit from high PR links. That could also make sense to me as many sites have gone down with no appraent reason. Also is there any sense to the theory that there has been a general down-valuing of PR? I guess this thread is for people with a few sites as it's impossible to draw any conclusions from just one or two sites.
I dont think google is devaluating high PR links, one of my website which had absoluetely no backlinks and not much SEOed went onto PR6, it just had one PR7 link pointing. But some of the websites which were focussed on did not manage to gain PR. This update has been strange for many reasons.