You can skip the NAT traversal in all protocols, so especially VoIP, video conferences and games will be less expensive, more reliable and simpler. And incoming connections will be possible, meaning all that smarthome stuff could get simpler, more reliable and more protective of users' privacy.
Of course contacting a webserver will mostly be the same. Maybe some loadbalancers will be less confused, but I doubt an enduser will notice.
are ISPs going to use https://en.wikipedia.org/wiki/Mobile_IP though? because with a roaming ipv6 address you will have a device identifier that you may not be able to change even when roaming. advertisers couldn't have wished for something better.
When people say pulbic IPs are "unique" they mean in the set of public IPs the same IP won't be handed out to two different entities not that each end user gets an IP that identifies them uniquely. There are only 2^32=~4.3 billion IPv4 addresses (a very good chunk of which aren't even publicly routed on the internet) so even if you wanted to it'd be impossible to assign IPs in such a way given the number of users on the internet.
IPv6 doesn't allow you to make this assumption 100% of the time either even though it has 2^128. A /64 is not permanently assigned by an ISP - especially true for mobile services - nor is every end network guaranteed to be a /64, it was just a best practice recommendation. There could be thousands of /127s in a /64 instead. Same can be said of NAT66, it's discouraged but I'm sure some ISP somewhere will do it.
Duplicate IPs for accounts can be one flag to help you look for other signs but it should be by no means proof of anything on its own nor a sign anything funky is going on with a region's internet.
NYC has a ton of street level internet kiosks and open wifi endpoints around, it’s common to have housing above store levels, and it’s common to freeload on them if you are lucky enough to be near one. That might be what your seeing, and as others have mentioned anyone using a cell phone is probably coming through a NAT gateway unless you support IPv6 on your edge.
Personally I’ve not been able to connect wirelessly to any of the street kiosks since about the second week of their existence. I still see the SSIDs advertised but I can’t negotiate a connection. I don’t know if that is because they are congested, broken, or that feature is switched off. I only have one near me but you can find them every block in some parts of Manhattan.
I wonder if they ban MAC addresses that spend too much time on the same kiosk for exactly this reason, to prevent people (who don't know how to work around it) from using it as their primary internet connection.
The purpose behind these is obviously advertising and I'm assuming that the most valuable data they are after is physical movement data and that it is valuable enough to subsidize the costs of running the system. However, if you don't provide them this data (by only using the service from a single fixed location) then they have nothing to gain from serving you.
1) They could be using café wireless from different cafés that are backhauled to the same data center (or LinkNYC as another commenter mentions.)
2) They could be using residential IPs, which are frequently reassigned. When my router restarts, I usually get a new IP address from the ISP's pool. This is more common for some ISPs and locations than for others; at my last place, we had the same IP address for 5 years.
Are they using LTE? We ran into a weird edge case issue a few years ago with Verizon wireless. They NAT and inspect everything, including rate limiting traffic the fit an abuse pattern. Weirdness happened more if the user was transmitting many packets when trAnsitioning between towers.
It could be related to the volunteer/non-profit mesh network ISP nycmesh.net, another ISP NAT'ing connections.
It might have an unusual apparent topology, with mostly to entirely wireless backbone links based on line-of-sight. Although I think users you talked to would identify this possibility, e.g. your moderator.