Nick Wilson (whom I recently had the pleasure of meeting at SES, though he was a bit jetlagged from what I gather) is asking folks what they want from an ad network. To me, the answer is one big item, and many smaller ones.

Relevance - it's the most critical metric for an ad network because it defines how successful the network will be overall. Every ad that is highly relevant to the content on the page will boost its value and every off-topic ad will hurt it. Google isn't the market leader with AdSense because of their customer service, their empowerment or even their size (Yahoo! is quite nearly as large). They're winning based on two items - payouts and relevance (with the first being directly tied to the second).

I've had a lot of discussions in recent days with folks about whether AdSense is a natural monopoly - the prevailing wisdom seems to be that someone has to go far beyond its current relevance to compete. There's two ways to do it - algorithmically (which I think would be very hard), or humanistically (allowing folks to select their own ads to display based on a very clever taxonomy). What do you think? Is AdSense beatable? Or should Nick, Threadwatch, Yahoo!, MSN & Ask throw in the towel?

UPDATE: Seems Darren Rowse is in agreement and has some tips for what Google can do to improve the key metric.