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Keep it Simple.
In part as a reaction to the complexity of Multics, the "Keep it Simple" principle started at the very beginning, and remains true until today. As an example, Windows Server 2008 requires 32GB of disk space for a minimal install, while a modern Unix release such as AIX 6.1 will happily install in less than 3GB.
"Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new features."
The above quote is taken from a paper by Doug McIlroy, one of the early fathers of Unix, on his article "Unix Time-Sharing System Forward", published in The Bell System Technical Journal in 1978. Any Unix user who ever combined two or more simple commands with a pipe can attest to this.
And more...
For an in-depth immersion in the Unix philosophy, please take a time to read Chapter 1 of Eric S. Raymond's "The Art of Unix Programming": Basics of the Unix Philosophy