Today's user interfaces are inconsistant. A command run in one application may have no relation to the same command in a different app. Even the way to run the same command in two different apps can vary widely (ie X windows).
Users are dependent on developers to provide decent interfaces for their applications. There is nothing they can do to make an ugly program look better.
The whole concept of applications and linear flow of control, on which many OSs and applications are based, goes against how the human brain works. These concepts force a good user interface to use abstraction, to hide the "real" system from the users. If the underlying OS had been designed well from the start, you could present them with what was really there.
Current UI APIs are monolithic, not broken down in smaller, more useful and replacable components.
Many OS/UI vendors, such as Apple Computer, provide rigerous standards for an application's look and feel to conform to, but provide little or no software assistance to developers for implimenting such standards.
Due to the non-OO nature of most UIs, a helpful (UI) subroutene developed in one application may require changes in the logic of another program, should this other program's developer wish to incorporate it.
Using = programming, just in a real-time sort of why. Users aren't stupid; they just don't have time to muck around with all the silly details of programming. For them, we should have easy-to-use scripting language, for intelligently automating stupid tasks.
Every component in the system--seen from both the user and programmer mindsets--should be replacable. This, done through object-orientation, will make replacing individual, outdated pieces of the OS as painless as possible.
To make different copies of Tunes compatible with one another, there will exist a set of standards that all Tunes systems should support in some way or another. Such standards should allow for IO of a number of different data types.
These standards should concentrate on specifying what should be done, and not how to do it. Instead of specifying the interface with which a user can input a certain data type, a program would ask the interface for a certain type, and it would input it in a user-selectable way.
The APIs will be modular, so that you can access and interface with only the parts that you need for a certain job.