Mac OS X preferences -were- human-readable (and editable), but that's not what makes the NSUserDefaults API so handy. What makes NSUserDefaults handy is the support for varying default domains (network, system, user) and dead-simple serialization of Objective-C basic types (strings, ints, dictionaries, arrays).

Additionally, Apple switched to a binary file format to improve performance in 10.5, which may be converted to/from the text format, or edited directly using the property list editor.

There's also nothing stopping a software author from documenting their settings file keys, and some do. However, most of the settings options you do see listed on that site are undocumented because they are unsupported.