Thank you for an outstanding and interesting response. I apologize, as I was mostly focusing on differences within the set of industrialized countries, a la The Spirit Level. I should have made that clear.
Now, the Great and Powerful Wikipedia is telling me that the PPP GDP/capita is $48K for the US and $35K for France. (PPP vs. Nominal is an important distinction, one I wasn't really thinking about, so good catch there). If correct, that's 37%, not 10%. The difference seems fairly stark, though it's not clear to me what that extra 37% purchasing power is buying us, since both countries are plenty wealthy enough to provide for their people, and France seems like a much nicer place to live.
I don't have any data to back up my statement that France could close the GDP gap by working longer hours. It makes intuitive sense to me, though my model is probably overly simplistic. The argument here reminds me of the (problematic) Laffer Curve. There has to be some point where working an additional hour actually diminishes the quality of work to the point where you're actually less productive over the entire labor period.
Extreme example: Say I'm working 154 hours a week and getting two hours of sleep each night (the minimum amount of sleep Navy SEAL trainers are required to give trainees, IIRC). Now move one of those hours a night from the "sleep" column to the "work" column. At that point, it doesn't matter what the nature of the work is, you're going to be way less productive at it.
Like the Laffer Curve, the actual shape of the Work/Life curve varies tremendously from person to person, by working conditions (if your work is inherently rewarding, or extremely hard on the body), by life conditions (if you're in a bad marriage, work might be where you go to unwind, to feel useful), whether a change in hours comes from vacation time or a longer work day, and probably by a dozen other factors that aren't coming to mind. So it's impossible to say which side of the "traditional" 40 hour work week the ideal falls on, even as a society-wide average. Maybe I'm too hung up on 40 hours a week as "the norm," but I suspect that adding a few hours to France's work week would result in increased GDP. If my math's right, and you assume the additional hours were as productive as the original hours (probably not the case), it would lead to a 25% increase in GDP, significantly closing the gap.
Which is kind of suggestive to me. Perhaps at this point in the evolution of the economy, we should be trying to maximize GDP per hour of labor, instead of GDP(PPP) per se, with some mechanism to keep Gini from straying too far from the ideal.
Quality of life metrics are indeed somewhat subjective, but I think a rough consensus can be obtained. For example, self-reported happiness is certainly a better measure of quality of life than, say, average educational attainment or per-capita hours spent playing video games. The Spirit Level makes the attempt by taking the unweighted average of several different indicators, which seems like a good start. It seems incomplete, because they only included those indicators that they found had a statistically significant relationship to inequality (educational attainment, drug abuse, obesity, life expectancy, levels of trust, etc.), and while they avoided weighting in order to avoid making value judgments, I think you could dig in and select some weights that make more sense than the unweighted version.
Your dad makes an interesting point. What do people do when money is no longer a big factor in their decisions. I assume people are generally more productive when they're doing what they're passionate about. Say you could become a doctor, which inspires you, or a job on Wall Street, which sounds like a boring job that would add nothing to the world. If you can make $80K/year as a doctor, and $110K/year on Wall Street, it seems like a pretty obvious decision. But if instead you could make $1.1M/year on Wall Street, suddenly "following your passion" makes you look like a chump. It's a good argument for a strongly progressive income tax; if the government is going to take 75% of your Wall Street dollars back, there's little incentive for Wall Street to offer those economy-distorting salaries, and less incentive for workers to take them, freeing the best and brightest to do what they love, and freeing up seats on Wall Street for the gambling addicts who would actually enjoy the work.
In a similar vein, I'd offer a guaranteed minimum income. It wouldn't be enough to live on, unless you're happy sharing a cramped apartment with three other adults and eating a lot of potatoes. But it would be enough to force employers to offer more than just a subsistence paycheck. Just being able to say, "I need this job, but I don't NEED need it" would be enough to give laborers more leverage against poor working conditions, and more freedom to find jobs that they actually found fulfilling. Plus it would be a humane incentive to automate away the boring, unrewarding tasks. Side note: I think that if the US existed in a vacuum, and we couldn't outsource our labor needs to the developing world, we'd be much further along the path to a mostly automated economy.
My thoughts. Take 'em or leave 'em.