Good programmers know what to write. Great ones know what to rewrite (and reuse)
If you treat your beta-testers as if they're your most valuable resource, they will respond by becoming your most valuable resource.
Any tool should be useful in the expected way, but a truly great tool lends itself to uses you never expected.
Complexity control is the central problem of writing software in the real world
Release early. Release often. And listen to your customers.
If Unix could present the same face, the same capabilities, on machines of many different types, it could serve as a common software environment for all of them.