Despite its massive popularity, Google have failed to capitalise on YouTube's audience. The trial of its InStream overlay system has reportedly not being very successful and the company is now adding post-roll ads to its clips.
The point-and-click format is probably the reasons that YouTube is so difficult to monetize; pre-rolls and overlays are intrusive, but without a playlist capability, the instream option - by far the most successful format for internet tv advertising - isn't a possibility.