This is great. Also, for a more general overview of the theory check out the book Permission Marketing. It's largely about what gives large email lists their value, how to grow that value, etc.
My understanding is that sites like DailyCandy, etc. do well by having very good open/CTRs for their emails and known/attractive demographics, and can therefore charge quite a bit to place ads. (They don't list ad unit prices, but some more info: http://www.dailycandy.com/mediakit/)
Utility based content is also a big one. They send content that is useful/provides new insight.
Wow, the sale price for DailyCandy is an eye-opener (125 million)