"On the playlist page click the + sign in the lower right corner of any video thumbnail and drop down the "add to" options. Select the playlist you want to move the video to. "

Now the only option is "watch later." You can't add videos to a playlist from any thumbnail anymore.

Google, why do you keep making the youtube interface worse and worse every day? I mean, why? Do you just like to mess with users because you can? Is there an anti-social 14 year old running your user-interface team? What?