Online video is still very much in the “one of these days” mode — people draw the curve of online attention vs. online spend and feel there is inevitable massive growth. At the same time we see consumers increasingly willing to pay money in various ways for premium content. The monetization channels for video are just now starting to feel like they have a path to “maturity” — and many of them are just getting started (even Google's efforts are fairly new), and the media buyers are notoriously slow to adopt. In short, it simply takes time to get the buyers comfortable with any new format, and the confounding influences of all the talk of new ways to measure engagement and rapid pace of change relative to the previous several decades create a lot of friction to rapid growth, but much of that could credibly be said to be “almost over” in terms of making the transition. If you buy that, online video is poised to be a substantially bigger market in just a handful of years.

“Marketplace” seems to be defined very broadly here — look forward to hearing where you draw the distinctions.