Youtube embed ‘list’ parameter is apparently supported on the AS3 player only according to here.
In other words, regardless of embed method in WP 3.5.2 (iframe, object or WP built-in in-line post embed), Youtube playlists will only play on browsers which support Flash. On browsers where Youtube uses the html5 player, the playlist doesn’t work.
Try embedding this for example: http://www.youtube.com/watch?v=HPr7_tr84zY&list=PLq3_zC_Xz0PgHlZdL6_aljNR1_UxCGGdr&index=1
I wonder whether the new video core functionality in WP 3.6 based on mediaelement.js will play Youtube playlists on flash-less browsers (like Safari or Chrome iOS) using the list parameter?