Welcome to MUMSnet
The Register commentards are usually a sound group, but whenever the subject of IPv6 comes up, the comment section turns into something from Mumsnet or the Daily Mail - lots of loud uninformed opinions based on "research" that would embarass a flat-earther.
It's obvious that many of the complainants have looked at the address containing colons and hex and decided it's too hard.
Here are some facts:
IPv4 addresses are 32 bit - part of the leftmost bits are a network address, and the rest are either a host address or a further division of more net/host splits. When you get down to the final local network, the rightmost bits are associated with the host on the LAN.
IPv6 addresses are 128 bit - part of the leftmost bits are a network address, and the rest are either a host address or a further division of more net/host splits. When you get down to the final local network, the rightmost bits are associated with the host on the LAN.
The only difference apart from size is visual - conceptually they are the same : "net/routing part" and "other part"
Now, it seems many people want an expanded address range, but don't want longer numbers. How would you do that?
A 128bit IP address in the IPv4 address style would be a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p
I suppose you could say "for presentation, allow stripping off of leading zero octets. And map current IPv4 addresses to 0.0.0.0.0.0.0.0.0.0.0.0.a.b.c.d
Well, IPv6 does just that, but better - each address is broken down into 8 16 bit numbers, separated by a colon. Not only can these 16 bit numbers (represented in hex) have their leading zeros stripped, but groups of 16 bit numbers as zero can be replaced by "::"
Still, if you want, you can still use IP4 notation, e.g. ffff:02::18.104.22.168
You can use anything appropriate - remember, all it comes down to is a way to specify a 128 bit number, but just as IP4 can also be represented in different ways: (localhost: 127.0.0.1 or 127.1 or even just as a 32 bit decimal number: 2130706433), so can IP6, but you tend to use the form most appropriate.
tl-dr: It's just a bit number. 128 bit instead of 32 bit. No other difference.
There are LOADS of useful enhancements with IPv6 - but they are optional.
The only real "change" from IPv4 is that arp has been replaced by a more network efficient format that can also do other host discovery and duplicate address detection. - ICMPv6 is used for this.
This makes it easier going forward - one protocol instead of mixtures of arp/icmp/ as with ipv4.
This is only the real difference to IPv6, and it's a simple concept to understand.
IPv6 still allows you to use protocols that are no longer necessary if you want, e.g. NAT and DHCP.
You also have a large choice of transitional protocols to choose from: 6in4, 4in6, 4over6, 6over4, 6to4, nat64, dns64 -- none required, but there if you want.
As for "plug in" compatibility, many commentators are like the special kid trying to hammer a square peg into a round hole.
How are you going to fit an address longer than 32 bits into a 32 bit address?
The stack HAS to change. Data structures will be different. The only ways of encapsulation ip4 into ip6 already exist.
As for routing, IPv6 can already be encapsulated and routed over the existing IPv4 infrastructure, but guess what? It doesn't happen much because most of the wide are networking has already been upgraded, due to these systems constantly being improved. The issue with compatibility is not the networks or the routers, it's the endpoints. The users equipment. And no protocol with an expanded address range can magically make that transition invisible, though of course, with an ip4-ip6 conversion at the edge, ipv6 can still allow non-ipv6 hosts to connect to others as much as could be technically possible.
It's almost as if a bunch of experts have slaved over this for years, and thought of - and mitigated against - many things most would never even consider.
Still, what would they know? The real experts are el reg's very own mumsnet flatearthers commentards whose expert opinion is "I haven't looked at it, it's too hard", or "the addresses are too long"