When YouTube simplified the user interface and replaced ratings with two buttons for liking or disliking a video, the only information about the popularity of a video was the number of views. To see how many people liked or disliked a video, you had to click on one of the two buttons.
Now you can see if users enjoyed a video without having to rate it: YouTube shows two horizontal bars for the number of likes and dislikes. Some might be worried that visitors could be influenced by the existing ratings, but I think they're useful and YouTube should find a way to convert the old ratings to the new rating system.
The reason why YouTube dropped the 5 star rating system was that 5 stars dominated ratings. "Seems like when it comes to ratings it's pretty much all or nothing. Great videos prompt action; anything less prompts indifference. Thus, the ratings system is primarily being used as a seal of approval, not as an editorial indicator of what the community thinks about a video," mentioned a YouTube blog post from 2009.