Google is doing something that might have other ad networks in a tizzy by announcing that only ads that are viewable (50% of the ad for at least 1 second) will be paid for by advertisers.
From the Inside AdWords blog
Today, we’re taking an important step towards this goal by making it possible to buy based on viewability — in real time — across the more than two million sites in the Google Display Network. Viewability was already available for reservations buys on the Google Display Network, now this solution is available in the auction on a CPM basis globally as well, across desktop, mobile and tablet. In other words, you can now choose to pay for ONLY those impressions where your ad has a chance to be seen.
Google really has a chance to change how advertising is bought and sold if this works well and it could put real pressure on the rest of the industry to do as well.