Source: I founded a major display ad tech business.
(What about Google's prediction of $100B by 2017?)
I believe there are a few key things that need to change:
Publishers need to wake up and stop forcing buyers to use old school hand-to-hand combat direct sales methods (where 70% of the current market $ is). Make it easier for people to buy from you, and more people will buy from you!
Transactional friction must decrease. It is 10 TIMES more expensive to spend a dollar on a TV ad as it is to spend a dollar on a banner ad. (3 cents per branding dollar for TV, 30 cents for web.)
We need measurement practices and attribution other than the click through rate, which was good for performance marketing but bad for branding. The big budgets want something better in order to track their spending.
Engaging but unobtrusive creative formats. "When's the last time a banner made you cry?" Sight, sound, and motion on TV are still far better than banners at eliciting emotion. This needs to get figured out in order to enlarge the market.
any hiccup in multi-device, multi-channel attribution, like opt-in default cookies, privacy laws that hinder Data Management Platforms from integrating third party data like credit card purchase data, etc...
or some killer new native ads that change the face of display.