Our ideology can be described by the following guidelines
Do not add new functionality unless an implementor cannot complete a real application without it.
It is as important to decide what a system is not as to decide what it is. Do not serve all the world's needs; rather, make the system extensible so that additional needs can be met in an upwardly compatible fashion.
The only thing worse than generalizing from one example is generalizing from no examples at all.
If a problem is not completely understood, it is probably best to provide no solution at all.
If you can get 90 percent of the desired effect for 10 percent of the work, use the simpler solution.
Isolate complexity as much as possible.
Provide mechanism, rather than policy. In particular, place user interface policy in the client's hands.
From Scheifler & Gettys: "X Window System"