I really dislike the Dewey Decimal System. I used to think it was a brilliant system, but I've spent too many years in well-organised scholarly libraries (my college library uses Library of Congress classmarks and a home-brewed classification system) and gotten attracted to non-Dewey systems. My local public libraries use Dewey, and it is constantly irritating to me.
Currently, the 1xx region is for "Philosophy and psychology", but within that it's a mess. Psychology (15x) and "Paranormal phenomena" (13x) stick out like sore thumbs. There's an easy way to fix this. Move them to the end of the 1xx range - have Psychology as 18x and Paranormal phenomena as 19x, and shuffle all the philosophical topics down into the first seven ranges. If you were up for more radical change, then perhaps take Psychology out of the 1xx range and put it in the 3xx range - the social sciences. You could do that by compressing 35x and 36x.
In my local public library, there are a bunch of books on philosophical topics, but they are punctured with large amounts of cod psychology (of the Men are from Mars, Women are from Venus type) and paranormal claptrap. The Dewey system's 1xx range was probably fine for an age of Spiritualism and Phrenology (the latter getting it's own third-level category: 139) but in times where the cultish nonsenses are far more political and explicitly religious, the placement of 13x really does stand out. Similarly, now that psychology flourishes itself as a science, rather than as something of a latch-on to philosophy, it seems very strange to have the two mixed together.
This applies just as much to Library of Congress though - BF (Psychology) sticks out like a sore thumb in the B category. For religious topics, I'm a big fan of how LC do it though, and I could probably tell you down to a pretty good guess where you'd find stuff in the latter part of the B range.
In the end, it kind of makes you want to do it how some German libraries do it - have no categorisation scheme, and just put the books on the shelf based on publication date. Bring on the Semantic Web!
