Google added more features to the "plus" button in YouTube displayed when you mouse over a video thumbnail. Until now, the button allowed adding videos to a queue to play them later. Now, if you click small arrow next to the button, you can add the video to your favorites or to one of your playlists. Another change is that the "save to" drop-down has been replaced with a button that adds the video you're currently watching to the queue. Click arrow next to the button and you can add the video to a playlist.
When you add a video to the queue, YouTube shows a small bar that lets you manage the videos from the queue, save them as a new playlist, disable autoplay or load one of your playlists.