Air pollution: Caused by what? Probably cars.
Noise pollution: Caused by what? Probably cars. (Sure, some sirens here and there, but rural and suburban living doesn't make you immune to the sounds of train crossings, highways, jake brakes, motorcycles, etc).
Annoying neighbors, smell/hearing through walls:
1. Easily mitigated through smart building design and practices, enforced by building code.
2. You can have completely detached single family homes and still build walkable transit-oriented suburbs just by sticking to some simple development practices:
- Banning subdevelopments that disconnect form the city street grid, requiring residential neighborhoods to fit into a holistic city plan rather than farm parcels being sold where the city greenlights residential development projects without an ounce of thought to how it integrates with the town/city.
- Prohibiting neighborhood designs that increase walking distance, like winding roads and cul-de-sacs.
- Building traffic-separated, safe, pleasant cycling and walking infrastructure that offer a workable alternative to driving
- Building roads with traffic calming measures in mind. For example, unnecessarily wide suburban neighborhood streets encourage speeding.
- Properly funding transit service and removing social stigma against transit usage ("I won't take the bus that's for poor people" - we literally need government ad campaigns to help change minds on the issue just like those used to promote seatbelts.
- Prohibiting parking lots against streets, like in big box store developments. Parking should be required to exist in the back of the lot wherever possible.
- Removing mandatory parking minimums. Believe it or not, real estate developers don't naturally want to build parking lots that can't make a return on investment. They're often required to by the city to make them in order to...you guessed it...help ease traffic!
- Prohibiting excessive dead space/green space/setbacks in developments.
Overall, I don't agree that there are downsides to walkable areas that have to exist.