In terms of the game design, they wanted something to control growth of your empire. In previous civs it was partly economic (city maintanance and corruption sapped your income). No clue why they changed, but the patch changes announced make it clear they are stubbornly sticking to happiness to try to limit empire size, instead of gold.
Alpaca's and soon my mod will be using maintenance just like civ 4. But city unhappiness is still useful to make early game expansion slowed before you hit theatres, so we're using both of course.
Buildings can now no longer provide more Happiness than there is population in a city (wonders are excluded from this). (Added 12/3)
I am a little concerned about this. What happened to global happiness?
Less of a big deal than you think. It only prevents mass size 2 cities building colosseums, which actually wasn't very efficient in the first place. It does hit cities with theatres pretty hard, though.