All windows versions since 95 actually have pretty complex abstraction layers (IIDL -> user-space path, user-space path -> kernel path) between real kernel-level filesystem and what user sees in Explorer (To faciliate things like "My Documents" and also to provide illusion of drive letters and current directory). I assume that OS X's finder also has some abstraction layer (to do things like Library->Bibliotheque).
Also it is relatively simple for application on either system (including generic unices) to find suitable place for their data. Making some OS-agnostic abstraction has fundamental problem in fact that different systems offer different types of standard locations (Unix has $HOME and dot-files, while in windows you have My Documents and three distinct Application Data directories and lot of specialized directories like Pictures, Saved Games etc.).
Also it is relatively simple for application on either system (including generic unices) to find suitable place for their data. Making some OS-agnostic abstraction has fundamental problem in fact that different systems offer different types of standard locations (Unix has $HOME and dot-files, while in windows you have My Documents and three distinct Application Data directories and lot of specialized directories like Pictures, Saved Games etc.).