In my humble opinion it's the ability of the filmmaker to make you interested in the subject even if you're not particularly interested in it. I'm thinking Senna, Hand of God and many others.
Beep sadly turned a subject I am specifically interesting in and made it INCREDIBLY boring. Come on! The history of game sound, surely that's an easy one. Just give us loads of examples of the classic games we've played and interview the composers about their influences and inspirations. This docco was so dry and uninspiring and long - 1.50 and boy did it drag. In a strange quirk of fate I'd just finished listening to the audiobook of Ready Player One this afternoon so was in a very, very sympathetic space for some video game sound nostalgia, sadly unrequited.
One thing I did notice is that no one who was being interviewed seemed relaxed so maybe that was the vibe that the interviewer (if that is the same person as the director) gave off. No one loosened up and spoke their story in a compelling manner. This is like an educational video that insiders to the industry will likely only enjoy. I'm a gamer, I play old games and new games and I thought it was dull as hell. If you're not even interested in retro gaming then give this a wide berth as you will waste two hours watching it.