The Stack Exchange Sites list provides a list of all of the sites and some statistics about them: total questions, total answers, percent answered, users, visits/day, questions/day, and age.
These probably aren't the best metrics. For example, total questions and answers don't necessarily mean a whole lot to me. In most cases, I'm searching the site for the information I need. The total number of users is also relatively uninteresting, although a total number of active users (probably timeboxed and cached) would be more interesting.
Jon Ericson wrote a Stack Exchange Data Explorer query to report on the number of questions, median views, average score, closed rate, average answers, acceptance rate, answer rate, and median time-to-answer for a given site. Not all of these are appropriate for the Stack Exchange /sites list, either.
There are two aspects to this:
First, the /sites list should be updated. I'm not entirely sure what metrics I would display. I think that answered percentage is useful, as would be visits/day, questions/day, and site age from the current lists. Median time-to-answer from Jon's query would also be interesting.
Second, every site should have additional stats available. These should be cached, but storing things like total questions asked, total answers, percent answered, total accounts, active accounts, visits, questions, age, average answers, median views, median time-to-answer would be nice. These would be a whole lot more detailed and specific to one site.