The biggest criticism user make on YouTube is that it has no option to allow you to download videos. They believe that it will result in copy right abuse, while users believe that they should be allowed to save videos which they may watch later.
There are many other methods to download YouTube videos, many online websites are available which help you to download YouTube videos, such as savefrom.net.
Just visit any such website and paste the link of the video which you want to download, it will ask you in which quality does you want to download the video. After selection and clicking download the video download will start.
Recently YouTube also introduced a new feature in its official app from where you can save the video to watch them later, but that download is only temporary and after 24 hours the video will be deleted.