A while ago, YouTube changed its video rating system from a 5 star rating to a like or dislike rating. The reason given was that people mostly gave a one-star or a five-star rating, so why not go with a binary rating system? Sounds familiar... So, I took their word for it, until today when I found the actual reasoning behind it. Appearance-wise, the thumbs up icon is followed by the word "Like," which also appears on Facebook: this apparently signified a change in meaning from "I am anonymously rating this video" to "I am broadcasting to the world a social network activity information." In users' profile pages, there's a section that broadcasts, unwittingly or not, a bread crumb of your video surfing activities as well as your video preference to the world by showing which videos you liked. YouTube has also been pushing users to link their Google accounts with YouTube's, so that your friends and colleagues who know your email address can join in on watching the spectacle, popcorn and all.
The video rating sharing can be disabled under the "Activity Sharing" section of your Account Settings page (right now, it is: http://www.youtube.com/account?feature=mhum#sharing/activity). Or, if you feel like saying to hell with it, and want to delete your account, it is under the "Manage Account" section (http://www.youtube.com/account?feature=mhum#manage/delete).