Over on Coding Horror, Jeff Atwood writes YSlow: Yahoo's Problems Are Not Your Problems and makes some very good points about not taking YSlow results too literally--at least not without thinking about what you're doing.
But before you run off and implement all of Yahoo's solid advice, consider the audience. These are rules from Yahoo, which according to Alexa is one of the top three web properties in the world. And Rich's company, Topix, is no slouch either-- they're in the top 2,000. It's only natural that Rich would be keenly interested in Yahoo's advice on how to scale a website to millions of unique users per day.
That's good advice when it comes to following any set of recommendations. YSlow was designed for Yahoo's goals and will likely become more general over time. Take it's advice with a grain of salt, just like you should anyone's advice.
The comments on that post contain some useful nuggets as well, including some discussion from Steve Souders and the YSlow creators.
Yahoo! Developer Network