I disagree. Yes, nobody uses sendmail anymore, filesystems vastly improved and X isn't a resource hog any more. I've reread it some months ago and the main points still hold.

For example the part about `kill`:

    Most operating systems have a “kill” command. So does Unix. In
    most operating systems, the kill command kills a process. The Unix
    implementation is much more general: the “kill” command sends a
    process a message. This illustrates the first Unix design principle:
    • Give the user power by making operations fully general.
    The kill command is very powerful; it lets you send all sorts of messages to 
    processes. For example, one message you can send to a process tells it to 
    kill itself. This message is -9. -9 is, of course, the
    largest single-digit message, which illustrates another important
    Unix design principle:
    • Choose simple names that reflect function.