Though I am the greatest breaker of this rule, I agree so much with it.

Anytime I see a "Facebook killer" that has all the current features of Facebook I smile and tell the builder(if he/she is a friend) to observe that the sites after FB that ever got so big (Twitter, Instagram, Foursquare) had only one real feature before building out over time.

I am sure even Pinterest will become more complex, but for now, it is all about Pinning.

Nice article Joel, this point cannot be emphasized enough.