I would disagree somewhat on his stance that video quality is not affected by container format (especially on part "Here is a list of things that people commonly associate with a video's quality"). Different container formats have different limitations regarding what video (and audio) formats they support. And while it subtitles support doesn't directly affect video quality, it does do so indirectly. If you cannot add subtitles without hardsubbing or subtitle formats are so limited that you end up needing hardsubbing anyway then the choice of the container format ends up affecting the video quality.