It is interesting that Google has waited this long to prompt users to link their YouTube accounts, when Yahoo did it almost immediately after their acquisition of Flickr. Flickr users did not take to the idea very well, but maybe this delayed push will make the transition smoother for YouTube users.
As Google hints to in the transition message, linking YouTube and Google accounts is part of a larger push to integrate functionality of other Google services with YouTube. Looking at Google's master list of services, some likely candidates for integration include Blogger, Google News, and Google Talk. Fellow CNET blogger, Josh Lowensohn, seems to think that Google Finance might get cozy with YouTube. I'm not as convinced.
To me, the most exciting integration would be Google Talk. We are already starting to see a chat room scenario with Streams, but I would look for more chat capabilities to spread throughout YouTube, powered by Google Talk, as a result of account linking. Google News integration also has the potential to be extremely interesting. With videos on current events flowing back to Google News, from YouTube, Google has a killer platform in front of them to be a leader in citizen journalism.
YouTube already allows you to load contacts from a Google account to add as friends, so it's not immediately clear what Google plans to do with the account link, in terms of contacts. Hopefully contacts become a more central focus of YouTube, with a more robust messaging platform, since right now they are sort of hidden in the background.
While linking YouTube and Google accounts might seem innocent enough, under the disguise of making it easier to login, it is clear that Google has bigger plans for the new stream of data that this opens up. What Google services would you like to see integrated into YouTube?