I look at the uniqueness of the directory theme (usually shows how serious the directory owner is, after all if he/she is using a free template it may mean that the owner is such a cheapo to actually be serious about it), the pricing as well as how well the directory is indexed. Not necessarily in that order, by the way.
I follow a strict standard usually like Dan does at info vilesilencer. If the owner can't be arsed to - Make a clean looking directory; Make a unique directory; Add something other than empty categories; Add links themselves to at least not look like a PR farmer; Edit link titles and descriptions so they don't look like a duplicated keyword festival; Then I can't be bothered to submit my sites there