Most SEOs know that sites, as a whole, can eventually lose their trust from Google to pass link juice and PageRank. Sometimes this is done on a sitewide basis and sometimes it is done at a more granular level. It is probably also done both on a manual action basis as well as on the algorithmic basis.
With regards to PageRank, I think the main issues that we see there are really if we recognize that this is a site that it doesn't make any sense to pass any PageRank from. Then, on a site level, we might say, OK, we're not going to pass any PageRank from here.
That can happen, for example, if we can tell this is a site where people have been spamming their links for a long time, and it's maybe an open forum where all the links are followed, and it’s filled with clutter.
So those are the kind of situations where we say, well, we see this site is having trouble, maybe they can't keep up, maybe they're doing this on purpose. And we just don't want to trust it with regards to our PageRank calculations.
Here is the video embed at the start time:
Then Daniel Picken asked on top of that, "would affiliates and directories fall into the category of not passing any PageRank?" Which John Mueller replied, "not by design, no."
Forum discussion at Google+.