If only we had the knowledge to plan cities so that it's possible to live near where you work. Perhaps it sounds crazy, but we could even stack dwelling units on top of one another.
Seriously though: It'd be great if apartments were better sound-proofed. My only gripe with apartment living was inadequate privacy and being woken up at 3 AM by a lead-footed upstairs resident.
In places like Europe where more people live in them, they tend to do a bit better job of privacy, because they're not just 'cheap housing for people without the means to live in a "real" house'.
Yeah right, next thing you'll be telling me there's a better way to move large quantities of people than each one sitting on the highway in their own personal metal box