if a directory is pr7+ then other factors are pretty useless, you can say it is strong pretty easily! though i would say traffic is always appriciated, if a directory is pr less then pr6, but has a lot of traffic, and that too quality search engine traffic, i consider it strong
Any Dir which have Strong PageRank. And can pass good PR further. Inner Pages also have high pagerank and all pages are well indexed with no supplemental results in 'G'.
Here is how I view and determine if a directory is strong. High position in the SERP PR - PR5 or above and stays that for at least 1 year Dmoz listing Yahoo listing Quality backlinks - backlinks from good sites with high PR Inner page PR - at least PR4 in more than 20 inner pages
When I am looking for links to buy, I make a check to see if the directory (site) is "strong" I check PR then PS. Then I check its backlinks both number of them and quality. There is software that does this. A traffic research is also good and also the age of the domain has its role.