While there's a lot to debate in the article, kudos to Graeber for talking about a taboo.
White-collar employees 'work' (periods of focused concentration) a lot less than 40 hours. There are exceptions, and HNers may deny this fact (because the demographic is different or because the taboo is more powerful in this circle), but Graeber's inebriated cocktail party confessions match my own discussions with friends.
However... We've learned a skill, learned the job, and are available for 40 hours. Western culture really doesn't allow us to just show up for the 15-20 hours we really work and get paid the same salary as the 40 we say we do. So instead of going home early and cooking, working in the garden, bettering themselves, or watching tv, many show up to work and have strange meetings about nothing, conversations at the coffee corner, work unproductively on a tasks that could be done efficiently if they hadn't already been at work for 7 hours, or fuck around online while nobody is looking. It's great for eBay and Facebook, not so much for the kids in day care.
To some extent, everybody knows this. Employers are starting to tolerate working from home a lot more, which I think is a tacit acknowledgement of the situation. I hope that our culture can come to grips with this and acknowledge that most people in creative jobs can't really put in a solid 40 hours of truly productive work, while at the same time not condemning them to work as consultants with no savings or pensions.