i would love to live in a city where it was impossible to be more than 15 minutes walk from
a waitrose or M&S simply food for luxury groceries
an aldi or lidl for the cheap stuff
a mid-range supermarket
an independent green grocer
an independent butcher
an independent fishmonger
a deli/cheesemonger
a bookshop
a variety of stores selling gifts, homewares and clothing
a choice of interesting cafe's and restaurants
a bike repair shop
a row of charity shops
a swimming pool
an affordable/cheap gym
a luxury gym/healthclub/spa
a large park with great children's play area
and that there was plenty of affordable and spacious housing of all types and sizes such that all the people working in or needing access to all those businesses could easily live nearby whatever their family circumstances
and that each 15 minute neighbourhood contains one each of every possible type of school so that no one needs to go to school more than 15 minutes walk from home (and likewise the above housing condition would mean that no teachers had to live in a different area to the school they work in)
obviously the area where the hospital is would be allowed to be a drive away as you can't have a hospital every 15 minutes of walk, but the hospital neighbourhood needs to contain all the affordable housing needed so that all the hospital staff can live within 15 minutes walk, as well as plenty of large luxury housing for the consultants.
as it is, I live in a rather cheap down--at-heel area near the centre of a medium-sized city. within 15 minutes walk I have a few of the things I need. A lot more of them are about a 40 minute walk away, in an area where housing costs are about double what they are in my road - I do that walk occasionally if I need exercise, or do it by bike, but then am limited to what I can carry back so if I need more than a rucksack full I go by car, which is a 5 minute drive but it would be 45 minutes if I had to go out to the ring road and back in. The city centre is likewise about 30 minutes walk in a different direction, and I go by bike if I don't need to bring much back, or drive if I will be buying lots. The nearest swimming pool is rather skanky and grim so I drive to a much posher part of the city to use a nicer one (there's no bus route from that part of the city to this, and the hill is too steep for me to cycle)
the thing is though that these nice areas which have all the nice things within an easy walk can only sustain their businesses because they have a customer catchment area that is a lot larger than their local 15 minute walk radius. I would love to have duplicates of all those things within 15 minutes of where I am but I do not think the businesses could possibly thrive with only the customers who live less than 15 minutes walk away, and I am 100% sure that most the employees of those businesses live in the cheaper areas further away.
and I am lucky - fit and healthy enough to cycle, and with a lot of good things not too far away. there are vast swathes of the city where the only businesses for 40 minutes walk in any direction would be are a row with a fish&chip shop, a betting shop, an offlicence, a barber/hairdresser and a "massage parlour" and there is no way that all the nice businesses that make for a pleasant place to be would be able to set up shop there and make a profit.