A quality link is:
1) On topic (The page linking to your page is about the same main topic)
2) Ranked well for the keyphrase you are after (In the top 1,000)
3) Contains the keywords you wish to rank well for
4) Has high PR (PR 4 or higher)
I left out high traffic because that is irrelevant from an SEO point of view. But if you’re looking at the big picture that would be #5.