* Happiness index should go way up.
* The generation that is heading for retirement around now doesn't really know what to do with their time. If you actually work the full 40 hours a week, and you have a family, then, especially a few years ago, you simply didn't really need a hobby. You could zone out and watch some TV on saturday, work on the home on sunday, and monday is another workday. The problem is: these people end up miserable in their retirement because they don't know what to do with themselves. With only 24 hours of work a week, you WILL find a hobby.
* People spend more money when not working, and when money is being spent, the economy is much healthier.
* Convenience goes way up; with a 24 hour workweek, work is forced into moving away from a rigid work schedule: You can no longer declare that monday-friday, 9am-5pm are work times and all other times are off-work. That means that people are expected to not work during times which are still considered normal 'work hours'. This leads to expansion of work hours, thus, more convenience (things being open from 8am to 8pm or longer, monday to saturday). This is more relevant for europe; in the US, most things are already open a lot, many things even 24/7. This also means that things that are usually only open on workdays, such as the DMV or a bank or a doctor's appointment can be visited by people without requiring awkward arrangements at work to make time.
* Unemployment is solved overnight, because you instantly multiply the job market by about 1.4 (okay, it's not that much, but more than enough to stave off unemployment for a long long time).
* It in fact moves the work market dynamic back to a seller's market, which is GREAT: Right now things like automated delivery trucks aren't advancing nearly as fast as it could simply because unemployment is already an issue and nobody wants to deal with thousands of unemployed truck drivers. Automation (which is something we should hopefully strive for; what satisfaction is there in doing a job a machine could easily do?) SHOULD be something we encourage; in practice, it isn't.
* Good for gender equality, in the sense that it is much simpler to share child raising duties if the normal workweek is just 24 hours. You can now have a real career without having your kid be raised by your spouse or a live-in nanny.
* Good for job mobility, because you now have far more time to think about what you wish to be doing, and more time to do research and even interview for jobs you are curious about. If people start moving towards jobs they like, the theory is that they'll be more productive in that job, so it's good for everybody and great for population happiness.
* Traffic jams are cleared instantaneously, and in general this moves work environments towards accepting that colleagues aren't either 'all present' or 'all gone', so even if traffic picks back up, workflows will adapt much faster to accepting that people arrive and leave in staggered fashion.
Sure, moving towards an actual 3-hour workweek, especially in a different way vs. the move from 6 to 5 in the past (where EVERYONE got the same day off: Saturday) is a long process, but it's so obviously worth doing!