What, exactly, is YouTube selling?

That’s the question on every buyer’s mind in the wake of the company’s “brandcast” presentation in New York a few weeks ago. It wants advertisers to commit to yearlong, all-inclusive sponsorships of its fledgling channels for prices in the $5 million range, as well as a high $20 cost per thousand viewers for pre-roll. But it’s using proprietary numbers from parent company Google to guarantee audiences, and some buyers are balking.

The entire proposition behind the digital NewFronts was that ad buyers could confidently move TV dollars to digital, where they would find similar or greater penetration for less money while associating with top-tier content. There are even rumors that YouTube has a subscription-based service like Hulu in the works. But measurement is a sticking point. Imperfect as Nielsen is (and complaining about measurement is a team sport in the TV world), it provides a universal currency in a way that isn’t yet duplicated in the online world.

ComScore and Nielsen’s...