The original NeXTStep/OpenStep/GNUStep format, as far as I know, predates XML by several years, perhaps as much as a decade before XML 1.0 (i.e., 1988). Heck, maybe more, for all I know.

I think the XML format for plists dates from the early days of widespread XML adoption (1999/2000), and I suspect the idea was to move to something XML-compliant, but that that was perhaps the only motivation. This is really just a guess on my part, but it tracks with what I’ve observed. In other words, I think TomB’s observation that it’s just a plist serialized in a nominal XML form is right-on.

I don’t know anything about the binary format.

I personally consider the OpenStep format (sometimes called “ASCII plists” as contrasted with “XML plists”) to be preferable to the XML format because it is (1) more compact, and (2) more readable. Arguably the key word in both those points is “more”, since everything is relative.

The XML plist is essentially a serialized CF/NSDictionary. The dictionary is just a bag of key-value pairs, where the value can be any CF/NS type, including another dictionary. This makes it very easy to manage preferences, but the simplistic data structure gives you equally simplistic XML.

For truly horrifying XML, look inside an Xcode .pbxproj file, or an IB Carbon NIB file.