Google is currently updating YouTube and this includes the Data API v3 getting more additions like RSS push notifications, captions, and comments. This new YouTube API supports most features from the Data API v2. Video flagging will be added in the next few days so don't worry if you don't see this feature right now, it will come very soon. Just in case you forget, Google's YouTube team announced that they would be retiring the Data API v2 on April 20, 2015. That day has passed which means some apps will no longer be supported.