IMO, that would present you with a big problem of "overlap"; spelling mistakes, people trying to game the system by creating a unique category, etc. Could be solved with a lot of dictionary work, and if it took off could be very interesting, as it would build up a community interest structure.
43things.com - what the heck is it? A directory? A community? A wacked out version of h2g2? Good for fun browsing, but suffers from the same userbased problems as Wikipedia - only as good as the information that the volunteers put in. As much fun as it is to read about somebody trying to "frack a cyclon", it's not really useful.
If you used the "ransom note" effect on a standard directory, I can see that popular topics would soon weight the whole page, as people would start clicking on the bigger links, thus making them even bigger.
The reason that so many directories use the Dewey classification system is because it neatly orders all of the worlds subject matters into compact little data trees. If we want a logical directory structure, then we need to go down the Dewey (or similar) path. And while we can start to classify sites under multiple listings, that soon produces a horrendous amount of clutter that bogs down the directory.
So, instead of putting a "Pictures of my pet" site under software, pets, image hosting, and web sites, the Dewey system would put it in a tree consisting of Animals ¦ Domestic Common Pets ¦ Hobby Sites ¦ Pictures thereof. One entry, in a clear position, where anybody with half a brain can find it. If I'm in a directory, and come across an entry for a pet site under "software" or "web sites", I'm going to conclude that the system is wonky, and loose trust in it.
My gut feeling on the matter is that it depends on the ultimate purpose of our site. If we want to classify information, than a Dewey type data tree model is the way to go. If we want a stew pot of information, into which the user can dip at random to receive mad pieces of information, then the H2G2 or 43things ways can be best, as they present the user with topics they never would have thought of themselves.
Of course, part of the problem as I see it is that until your reputation takes off, all directories are basically acting as an intermediary step between the SE and the end site. So, the user types in "Cat care site", and your directory pops up as one of the choices. So, the end user can either click on one of the other listings in the SE and go straight to the cat care site of their choice, or they can click on my page and see a list of cat care sites. How do we make it worth the users while to add 2 clicks to their surfing experience? By providing a clear, concise choice of sites for them to choose from. Check the referrer keywords, run it against your own database, and dynamically add new choices to the page. Suggest "related searches". Highlight the most popular pages. Etc!