Whereas many programmers just *love* complexity and kind of despise simplicity.

So, for example, say I'm looking for a nice old barometer to fix up in my hall - I'd find Antiques at the top level of the hierarchy and (without having to click/expand anything) I'd look at the 20 or so subcategories listed under Antiques and I'd try clicking on Science & Medicine (Pre-1930) to see if there were any barometers in there.

The folder metaphor is worth thinking about. Graphical operating systems have long used this metaphor. And it works if there are just one or two levels. In a real filing cabinet you might put a folder within a bigger folder. You certainly wouldn't have dozens of nested folders, like I've got in Windows Explorer. It's crazy!

Nested folders is one of the reasons why people stopped using the Bookmarks and Favorites lists in their browsers and started using Delicious, because Delicious's tagging effectively gives you a single-level hierarchy, but with the added benefit that you can find something by looking for a number of different tags, or a combination of tags.

Yes, by using shortcuts, you can effectively put a file in more than one file system folder - but it's cumbersome to do and, again, it's another way in which the folder metaphor breaks down.