Though if one lived in a country where it didn't usually get hot enough to own an air conditioner and you had a prolonged heat wave you'd probably end up suffering quite a bit.

Yup, just as it sucks to live in a place that doesn't commonly do central heating and (above all) house insulation when it's hit by a cold spell. If you don't have heating and whatever warmth you've got leaves the house through the walls immediately, even 10°C can be really uncomfortable. (That's roughly what I'd consider my "cool enough to wear long sleeves but not cold enough to need a jumper" temperature, for the record!)

So yeah, it's all about what you're accustomed to and prepared for. (For me, 40°C would be a point at which I no loner care whether it's dry heat or humid heat because it's too much either way!)