"By attracting a commanding share of the search advertising activity, Google also has the best data with which to create equations that maximize the money it makes from each search. It turns out that picking which ad to display when is a subtle art that can have a great effect."
I've been working on effectively harvesting and choosing keywords to best power and adnetworks like Google/Yahoo/MSN. I believe it's much harder than simply producing good search results. Search results are targeted at people explicitly looking for information, so in that sense it's like an auto-generated yellowpages entry, people want to click on something. The adverts on these pages benefit from the explicit nature of the search.
How do you do the same for display advertising on the rest of the non-SERP pages of the web? You get one shot at providing value, a bit like trying to hit the green from the rough and through a stand of trees. A subtle art indeed, and great fun to work on.