As per comment stream in Bug #711618, some users are confused when click to play is activated. Given that some users specifically want to disable some sites that others specifically do not want to, a shipped automatic whitelist is impractical. I would therefore suggest that the user is notified that some websites will cease to work as expected/intended while click to play is active and is given an option to whitelist common sites such as youtube immediately after enabling the option.
I don't think we need this anymore with the reworked click-to-play UX that is currently underway. That UX offers activating plugins long-term or short-term (i.e. while the user keeps using the site), so most remaining visible problems (like e.g. the youtube playlist use-case) will be fixed.