My view
"Google Analytics covers the majority of known and unknown requirements"
Gaining and analylising statistical information on how your visitors are using your site is key to improving the quality and relevancy of content.
The choice of statistical packages is largely determined by whether you own or outsource the hosting infrastructure, whether you have clear goals or contracted deliverables for the site and budget.
Urchin
Very flexible, about $3,000 for own infrastructure
Offered by some hosting agencies
SawMill
Log analysis system
Multiple sources
Great drill down reporting
Interpretation and action
Both Google and Urchin have easy-to-understand reports on visitor metrics and generic advice on interpretion. Choosing the right actions for changing content requires some experience and expertise.