Culture - The unhappiest countries (Moldova, which seems to have been a bit at sea ever since the fall of the Soviet Union, is a deeply depressed place) have a distinct lack of culture. Without culture there's no sense of identity, no connection to a country. No literature nor art means no sense of self, either at the collective or individual level.
Nature - Despite the general ennui that the Swiss seem to exhibit (from my brief, superficial observations), the country rates very high on the happiness scale. This is largely attributable to the very deep connection that the citizenry has to nature. Iceland, a stunningly happy (if very dark) country, also has this relationship with the outdoors. There's an appreciation, not a fear, of the land, connecting the people to the most basic thing that humans know.
Belief - It doesn't matter what you believe in, you just need to believe in something. Perhaps unsurprisingly, those who believe in reincarnation seem to be happier than those who don't (the Bhutanese for example). And those who worship the (false) god of ambition, you are forever doomed to be miserable.
Boredom - Happiness apparently requires you to be bored occasionally, which gives your mind some time to relax, to think, to reflect. Perhaps somewhat related, don't think so much (according to some very happy folks in Thailand and India, thinking is the root of unhappiness). So quit with the packed schedule and the deep contemplation!
Failure - The ability to try a variety of things and fail at them gives a person more freedom, and thus happiness, than just about anything. Again, Iceland, with its European-style social safety net, excels in this category.
Envy - Envy is the most toxic, unhappy emotion there is. Let it go, do not covet thy neighbor's anything.
Poverty - Don't be desperately poor. Money is not everything, as relatively poor countries like Bhutan show us, but abject poverty will never a happy person make.
And if I had to highlight one takeaway from the book? Move to Iceland. Who knew?