YouTube added more features to the "plus" button displayed when you mouse over a video thumbnail. Until now, the button could only be used to add videos to a queue, so you can play them later. If you click on the small arrow next to the button, you can add the video to your favorites or to one of your playlists. It's much easier to build playlists from search results and from related videos.
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 on the 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.