Google on web directory links has posted in his Webmaster blog about the importance of topical directories which are well moderated and structured
“Directory entries are often mentioned as another way to promote young sites in the Google index. There are great, topical directories that add value to the Internet. But there are not many of them in proportion to those of lower quality. If you decide to submit your site to a directory, make sure it’s on topic, moderated, and well structured. Mass submissions, which are sometimes offered as a quick work-around SEO method, are mostly useless and not likely to serve your purposes.”
We know that theme-based links from any source, it may be blog posts, forums, articles, or social media, always provide the best results for your directories, here same is explained by Google but only limited to topical or niche directories.
Let us see exactly how many directories on the internet are available which are topical and well structured, for example, directories on Webmasters and SEO niche where you can submit Search Engine Optimisation, Web Design, Web Hosting or Webmaster related sites, they are very less, I assume that they are near 50 which are good ones and again some of these are paid.
SEO and Webmaster is the most popular niche and directories can be easily launched because they are themselves webmasters in these concepts, however having this amount in popular niche means, you will have very fewer directories in any other niche. So it means topical directories are best but they are not enough on the internet to make a good impact on your SEO projects.