Good points on mixed use. I work in Toronto and commute about 1.5 hours each way (Toronto apparently beat LA recently for worst commute time). I train/bus/walk it but with one bedroom condos going for 3-500k versus 2000sqft houses on 50' lots in the burbs it just doesn't make a lot of sense to live in the city (unless you desperately want the urban life which I don't: you can't realistically afford to raise a family in the city unless both parents are working professionals). NYC might also have been forced to be more reasonable because of its island nature. If you were stuck taking a fairy and then hiring a horse every day to get to work you'd quickly learn that if you stay on the island things get much simpler. Sadly Toronto has a lot of land around it so the "Greater Toronto Area" spans an area about 70km (45mi) in all directions outside of the borders of the city proper and silly immigration policies that allow ~40% of immigrants (not to single them out just for being immigrants but for being the source of population growth in Canada) to all move to this one city creating an endless pushing out of suburbs further and further away.
I'm not sure of a good solution to the problem. A random idea: charge people/corporations a fee per commuter mile. It would either force suburbanites to move closer to work/change jobs or push employers to move to less densely populated areas where mixed use is more prevalent, or even better which I'd like: make companies have to consider if having people commute into work to do a job they could easily do from home makes sense. I saw a Ted talk last year that claimed that cities were actually more environmental though because people generally do get around on public transit/walking more, live in smaller spaces etc. I'm not sure that it took into account that say roughly 30% of a cities population had to drive into it in the morning from much further away. Perhaps if the typical city was mixed use you would loose a lot of the efficiencies as you'd have a larger proportion of families requiring larger living spaces/schools/parks.
NYC isn't the biggest in North America though: that honor goes to Mexico city (both urban and metro areas are larger (about 200k and 2M difference respectively), then NYC, LA, Toronto.