Links Are Not Born Equal
Some links come from pages that don't help your SEO (described on the left). Some links REALLY help your SEO (those from PR4 or above websites) and others hurt your SEO (links from spammy neighborhoods or link farms).
Post Panda a link from a PR8 or PR9 website could hurt to if it is all alone. A single link with the proper anchor text from such a highly ranked website must have social signals. Without Facebook likes, shares and other links that PR9 site will stick out like a sore thumb.