- Pathfinding/multi-user interoperability. The ability to locate the appropriate directories for various things, such as machine-wide data, user-specific data (data that will be stored on the server if the user is on a domain), temporary files, shared programs, etc. I call this multi-user interoperability because it will allow (if used properly) programs to run as non-administrator/root (this is something most Unix people should know, but way too many Windows programmers don't), as well as work efficiently on a network.
- Power management. The ability to discern what state the computer and components in it are in, so that inappropriate operations (i.e. doing background stuff on the hard drive when the computer is on battery) can be avoided. I'd also stick Windows hierarchal storage management (the automatic archival of files to tape, and automatic recall on access) in this category, because it also suggests that things should not be touched even though they can be.
- Session information. The ability to tell when the system is shutting down, the application is going to the background, and if the application is running locally (on the computer the user is sitting at) or remotely (on a different computer, where the user's computer is just a terminal, and where bandwidth is often low, and latency high).
So yeah, welcome to the todo list, gang!