This is a great paper defining trading desks, telling you who the big players are and what are the pros and cons of working with a trading desk --- A centralized, service-based organization that serves as a managed service layer, typically on top of a licensed demand-side platform (DSP) and other audience buying technologies; manages programmatic, bid-based media and audience buying. Works as an agency's internal "center of excellence," supporting agency teams wishing to tap into this new buying model on behalf of agency clients.
Great discussion on GRP standard for online video - Here is the opinion of Tim Avila from Brightroll, "GRP matters less online. It's an insufficient measure in a realm where ad effectiveness can be determined more directly. In fact, better accountability is a well-documented reason for online video s torrid spending growth. In an interactive medium where advertisers can measure lift in online sales, keyword searches and consumer attitudes in near real-time, aggregate measures like GRP become less important. As online video spending continues to grow, look for GRP to matter less, not more."
What are the key characteristics of a true DSP? Here is a broad list of ideas from people in the industry, and direct quotes from agencies that have gone through this evaluation process.