Have you ever asked your self if submitting to web directories hour after hour every day have any meaning? And why, if you have many backlinks, the search engines still don't pay you much attention? Here is the secret - the "nofollow" on the links.
So what nofollow mean? Nofollow is a value that instructs some search engines that the link should not boost the link's target site rank. The main idea is a spam prevention as google people said in their blog - preventing comment spam. But like anything it can be used with evil intentions.
So what do you think happens when you submit a link to a web directory and it is tagged as nofollow? It doesn't deserve the time for doing it. There are thousands web directories that are not user or search engines friendly, they exist solely for the purpose to lure people to submit pages and don't give anything back.
Nofollow is not always a bad thing. Google recommends that paid link should have nofollow. But use it careful, and don't forget that people usually get frustrated from needless stuff.
Recommended reading for the nofollow attribute - wikipedia