It's just that we've got a double-dose of simplification here: not only is Happiness now global instead of per-city, but also Happiness now takes on the additional roles that health and corruption did, of limiting not just the size of cities but the size of your empire. So what you get are some very artificial effects: like when conquering a city, the occupied population seems fine with it (they will get to work right away as soon as the resistance is over, with no penalties compared to the rest of your cities), yet your whole empire suffers unhappiness.
I was really happy in Civ IV when they removed the "civil disorder" mechanic, because that was a pain in the butt, but it was still good that you might have unhappy cities, but it was localized: it was a city that you had let grow too big without the appropriate buildings, or had mixed ethnic groups, or had recently been conquered. There was nothing overwhelming about managing individual cities' happiness, and it added to the feel of how things were going differently in different parts of the empire.
I was really happy in Civ IV when they removed the "civil disorder" mechanic, because that was a pain in the butt, but it was still good that you might have unhappy cities, but it was localized: it was a city that you had let grow too big without the appropriate buildings, or had mixed ethnic groups, or had recently been conquered. There was nothing overwhelming about managing individual cities' happiness, and it added to the feel of how things were going differently in different parts of the empire.