Yahoo succeeded initially not because it was the first attempt at a large-scale Web directory (it wasn't), but because it was simple and well designed. For a long time it stayed this way, even if a fair bit of the simplicity -- the lack of graphics and fancy layout -- was accidental rather than intentional.
But sooner or later, when you start getting as popular as Yahoo you have to start doing anything you can to make money. And when you make money through the usual sources -- paid advertising -- you start getting less and less useful. And so it is with Yahoo today: in a decently-sized browser window, you have to scroll down past a fair bit of advertising junk and shopping links before you get to see all the categories.
Enter the ODP.
A volunteer-based directory was bound to happen eventually. I think the reason the ODP succeeded, where others such as OneMission did not, is for the same reason Yahoo succeeded while Galaxy did not -- the ODP was simpler and less strangely organized.
But still, the ODP has its problems.
The major advantage Yahoo has over the ODP is in the integrity of its classification. If an editor is a disinterested employee, they are more likely to compile a fair and representative list of links on a controversial topic than a volunteer editor is. They are less likely, for example, to pre-judge an organization for directory users by categorizing it as a `hate group'.
In fact, Yahoo's categorization tends to be better than the ODP's in general. ODP editors may be enthusiasts, but in most cases they sure ain't librarians. In many categories there are just far too many subcategories -- they are `flat' and `shallow' where they should be `deep' and `narrow'. But it can be hard for anyone but a ruthless librarian to put things in few enough categories so that users don't have to scroll to see all the categories on one page.
Then there's the matter of general balance. Whereas Yahoo's main categories fairly thoroughly and obviously cover the subject matter of the known universe, the ODP's main categories are rather eclectic.
Why are role-playing games listed on the front page of the ODP, whereas psychology is three levels down? Why is clothes shopping on the front page, while the environment is hidden away inside the `Issues' category? Why is there an `Issues' category at all -- who decides what is an `issue' and what isn't?
If I had the time to volunteer for the ODP, I'd ask if I could work as a general classification person, rather than as an editor for a specific category. Because a lack of decent categorization is the only thing which is really stopping them from being the ultimate Web directory.