The PR system works like a pyramid. PR0 feeds to PR1. PR1 feeds to PR2. And so forth. In combat sports, all you have to do to get a 10-0 fighter is to hold a 10 round tournament with 2^10 fighters. This is how it works in link building, each page of content is a 0-0 fighter(the power of content). Every link in is a victory over a fighter and every link out is a loss. Note that PR is different from ranking, in ranking both links in and links out count. The anchor text you use to link out affects relevancy(it's like a super bold). Ranking depends more on how many links are passing through(who have you beat?) whereas PR losses matter.
But, we're talking about page rank here. Remember each new page of content is a new 0-0 fighter for you to job out a link. If you get a whole bunch of 0-1 links, you're called a can crusher. Everyone is expected to have can links(in MMA fighters usually fight cans until 5-0 and Boxing until 20-0 and professional wrestling like 50-0). If a fighter is 0-100, then the fighters who beat them don't get page credit(a page has 100 links linking out). Whereas a 0-1 and 0-2 look pretty normal. If you have a blog and all it does is link to other blogs and sites and doesn't have links coming in it looks like a can. Google sees it the same way as a profile, directory, or bookmarking link. You need to job profile links at your blog to make it look less like a can and more like a gatekeeper. Gatekeepers are fighters who beat a whole bunch of cans(Travis Fulton is best example) and then lose to top fighters to make them look good.
You can get ground by beating cans(Rickson Gracie? 400-0) but you have to beat decent people to rise to the top. This is why it's important to link stack. If a page on the site doesn't have enough links, it looks like a can. You need to feed it can links until it seems like a gatekeeper :)