Since the very beginning, directories (of any kind) have had a very central role in the internet. (I have recently grown fond of Free Web Directory. Even Slashdot can be considered a directory: a collection of great news and invaluable user-generated comments. As far as software is concerned, doing a quick search on Google about software directories will return the free (as in freedom) software directories like Savannah, SourceForge, Freshmeat and so on, followed by shareware and freeware sites such as FileBuzz, PCWin Download Center and Freeware Downloads (great if you're looking for shareware and freeware, but definitely less comprehensive than their free-as-in-freedom counterparts).
As human beings, we probably owe much of the technological advancements to our constant need to categorise, order, and control—as well as our curiosity. Directories on the internet seem to be driven by the same desire and the same curiosity.
The result, as usual, are mixed. However, the two parameters which seem to make a difference are: user-contributed content, and the quality of the entries.
The best directories—whether they are about software, clothes, etc—are the ones which are enhanced by user input: people can give their opinion on a particular entry. This has made the fortune of sites like Digg (where users can vote and allegedly decide what goes to the home page) and Slashdot (where people's comments are the real value of the site).
Other directories, like the much criticised DMOZ, don't allow user input but concentrate on only having extremely high standards as far as submissions.
At Free Software Magazine, there used to be a software directory. It was way back in the very beginning, I very much doubt anybody remembers other than the original staff of FSM and me! However, the idea soon died out because the feature was lost when the site moved to a new CMS.
Here is a list of features I think would be necessary:
The list goes on and on. I am sure it would grow to something 50 times as long once development started.
Which other requirements would you add? Which free (as in freedom) software exists that does the job? Any ideas?