I was playing as Queen Elizabeth in the Earth large map scenario. I have now unlocked slavery and chose the civ. However, for some reason, I cannot rush the units/buildings by sacrificing population. I even already have 7 pop on London and I cannot even rush an axeman, when 3.2 did!

Is there any other hidden changes that I've missed?

It's not really hidden per se, it's just something people tend to take for granted. Slavery has a ton of other mechanics going on. And yes, if you're coming from 3.2, there's a ton of other changes. Basically almost everything is different, especially when it comes to specific balancing.