I would second all the mention of air-con, but as it happens, the house I'm living in right now has shitty air-con. Like, vents in the ceiling... you turn the air-con on to the lowest temperature you can get, and the house still doesn't cool down. It takes roughly an hour and a half to drop one degree Celsius lower than what it was. And it doesn't come out of the vents in any sort of force... you hold your hand up to the vent and you feel roughly the equivalent of two people breathing out slowly... it sucks.
So I already know I *can* live without air-con... that said, I value air-con as much as plumbing, but when push comes to shove, I'd rather have plumbing than air-con.
I'll also add another modern convenience I couldn't live without - energy drinks. Without energy drinks, I'm a zombie all day, even with coffee.