arikgub - 9:35 pm on Jun 28, 2012 (gmt 0)
the bad/good ratio is probably the most telling and easily computed one
Indeed, bad/good ratio is the first thing that comes to mind, but in my experience that's not the criteria used or at least not the whole story.
On the date of the Penguin update my eldest and the most authoritative site lost 50% of the traffic. The site was linked by almost every serious website in my industry and also favored by regional and national media. Sure, it had its share of spammy links like any site having that kind of visibility would, but the overall bad/good ratio was very low.
After Penguin, trying to figure out the nature of this update, I bought thousands of the spammiest possible links (xrumer, senuke type) to few of my weaker sites that had only a handful of links before that and used to rank for low competition keywords. Bad/good links ratio surely went through the roof, but not only that didn't hurt the sites, it even helped rankings. The only undesirable impact was the unnatural links notice in GWT, but the rankings actually improved a bit.
Overall, I am really puzzled by this update. Obviously, Google wants us to believe that spammy links hurt rankings, but I am not sure if there is anything more than FUD tactics to it.
That way or another, I would never waste my precious time on chasing spammers and begging for link removals, like Matt Cutts is suggesting. I really have better things to do.