Forum Moderators: open
1. pageviews per hour, day, month
2. key pages - a banner on mainpage vs a banner on page 10,000 > obviously mainpage get substantially more than that 10,000th obscure page.
3. sessions, uniques visitors, and returns
4. market diversity - general, topic specific
5. regional - percent US, Can, UK and sub-regional state/province clusters
6. for the Google savvy people PageRank
Considerations are more toward "PPI" models but just the same any serious company wanted banner exchanges "specifically" will want this info > at least to indicate a fair exchange.
You could calculate this and advertise stats in about us section with a mainpage link "advertise with us".