Blog Directories

Blog directories provide a listing of blogs. All of the blog directories have manual check in place to avoid blogs which are created just for the purpose of advertising or promoting products. I went through 6 of these and also submitted my site, some I got added and for one(blogflux) I was not added and for one I am awaiting reply.
What are the features provided by these apart from providing a valuable back link (I read somewhere today that back links from a site having google page rank of 4 and above are being sold for around $20)?
PS - rankwhere.com was used for finding the Google Page Rank (GPR)
Blogflux - GPR - 7 Blogs Listed - 25681
- Apart from the directory, blogflux provides button maker, feed reader, widgets for map stats, page rank checker, analytics. Directory can be searched using tags, language and country.

Blog Top Sites - GPR - 7 Blogs Listed - Not known
- Using the javascript embedded in the site, submitted blogs are ranked.

BlogCatalog - GPR - 0 Blogs Listed - 30788
- Browse by category and based on tags.

BlogRankings - GPR - 7 Blogs Listed - 10230
- Search by newest, popular and top rated.

Blogarama - GPR - 0 Blogs Listed - 39027
- Browse by category.

GlobeOfBlogs - GPR - 7 Blogs Listed - 52740
- Different ways to search such as search by birth date of author, title, location

Here is the traffic rank comparison as per Alexa


Permanent link for the above Graph
Observations -

1. Its surprising that couple of the blog directories have Google Page Rank of 0
2. Looking at the traffic more the features provide apart from the just categorization, more will be the traffic. When it comes to traffic blogflux is leading and it is the one provides many other provisions than just listing. On the long run can these directories sustain by just providing the categorization of the blogs? I guess some are already coming with interesting search options such as search by birth date of the author.
   
3. None of these directories making the data available through APIs. I guess when so much of effort is put to collect the data, if it is made available through APIs it can help apps and widgets around the blog directories. Otherwise these may remain mere directories for which people come to just submit the data.
   
4. Technorati claims that there 55 million blogs. But the maximum number of blogs that is listed in one directory is just 52740 that is one thousandth of the existing number of blogs. But these directories include even manual check to make sure that only serious blogs are listed. For example blogflux mandates that the blogs should have at least 5 posts and 7 days old.
5. All of the blog directories request for back link but do not mandate. But blogaram mentions providing a back link will increase the rankings of the blog.