It is common to use the Google
Page Rank (PR), to determine the importance of a web directory since high PR is what most webmasters are looking
for when adding a site to a directory.
The difference between a web directory and a search engine is that
a search engine spiders (scan) the web for sites to include in their
database, and they do not provide an indexable link back to your site.
Directories however, are mostly human edited, and sites are usually
reviewed before they are entered into their index. They also provide,
in most cases anyway, indexable links back to your site, and this is
what most search engine marketers are looking for in a directory, static
indexable links that passes on Google PR.
List of great General directories: