Soundproofing. It's a small fraction of building cost, but one that is most often omitted because short term profit is disconnected from long-term utility and benefit.

I've experienced several absolutely miserably loud neighbors. Talk about destroying my potential.

I moved to a house -- way more space than I need -- to get away from it. To end up with neighbors who blasted their car stereos -- think SUB-woofers -- all day long. In an unincorporated community where there was no noise ordinance and the county sheriff left it up to the individuals to sort things out.

Another necessary component: All you "libertarian", laissez faire people need to decide whether you are really, fully behind that idea. In which case, I would have no problem going across the street and putting a slug in each of their heads.

If I'm going to solve a problem, then once and for all.

Otherwise, come up with some effective noise control ordinances -- AND EFFECTIVE, PRO-ACTIVE ENFORCEMENT AND PENALTIES.

So... if you want "new cities" including "denser" living, fix your fucking noise problems.