Great technology companies have been built on advertising: Google, Facebook, Yahoo (in the old days), not to mention many a magazine, newspaper and television network.
I have always loved the operational side of ad networks. They require building and managing a systems whose data throughput and reliability requirements rival a financial pricing and trading system. I have managed several of those, and the parallels are quite strong.
What truly interests me in advertising, though, is the brilliance of great creativity.