I hope this is balanced carefully, and we do not go back to ICS tactics or other odd exploits. We will have to wait and see -- hopefully everything will be fine.
Based on each new city creating unhappiness at a greater rate than extra population in a single city would, I suspect that ICS is going to be hard to pull off, though not impossible because, as stated, each happiness building boosts your entire empires happiness rather than just the home city. This in turn means that ICS is going to require that you also build up your cities.