3. Sites should be relevant
Being relevant is about making people care about what you have to say.
Some information can hold up longer than other information but time has a nasty habbit of slowly chipping away at the importance of what you have to say (today).
Often website owners will bulk load massive amounts of content into a new site hoping that search engines and users will flock to them. They often look at short term analytics, see an increase in both users and search hits, and innacurately conclude that they've "done their part".
As time moves along, general interest in what you said inevitably wanes because without constant injections of new and interesting information the general attitude of most users is "So what? Do you have anything else to contribute?".
Because the Internet ecosystem generally favours new information it's obvious that site owners who want increasing amounts of new traffic should give increasing amounts of new information on their site. It's also obvious that increasing your information flow will give old users reasons to come back, and more importantly, new reasons to send their friends to your site.
Relevance isn't a "set and forget" endeavor. You may be relevant today, but to stay relevant you need to be relevant tomorrow too. What will you say tomorrow? It should be obvious.