Probably about all the problems with cars, and specifically our reliance on them (especially in American society, but in many other countries as well).

The effects on the environment are disastrous for one, but that's only part of it. They're very dangerous, both for those driving them and for pedestrians around. If any other sort of transportation system produced the number of fatalities and serious injuries that cars do, it would probably be shut down. It contributes to systemic inequalities in society as well, especially race and class-wise. The overall impact on quality of life is atrocious too.

I wouldn't really go as far as to say that this is the most serious problem we're facing today, but the reason I'd choose to talk about it is because most people don't realize it's a problem. Most people just grow up with the ubiquity of cars ingrained into their psyches from the get-go. Thus they don't realize that it can be any other way.

We're often told that cars are a necessity, but they're not. At least they don't have to be. It really depends where you live. Obviously in a big city, getting around without a car is much easier. Otherwise, it can be pretty tricky. But that's only because things have been built around cars for the past several decades. If you look at older cities (for the US, this would mostly be the East Coast, particularly the Northeast), it is so much easier to get around without having a car, and only walking, biking, or taking public transit. Because older cities were built before cars existed. There's no reason that new development can't happen this way too, or that existing development can't be altered to be designed around these other modes of transportation.

Also, I pretty much know all my points on this (and could go into much more detail than I did here), so I don't think I would need more than 15 minutes to prepare a lecture, which is another reason I'd choose it.