Things are going differently for IPv6. In thinking about why, I like Dan Bernstein's description of a "magic moment" for IPv6. It goes like this:
The magic moment for IPv6 will be the moment when people can start relying on public IPv6 addresses as replacements for public IPv4 addresses. That's the moment when the Internet will no longer be threatened by the IPv4 address crunch.
Note that Dan focuses on the address crunch. Despite claims to the contrary, I believe most people are interested in IPv6 for its very large address space. While there are other cool things in IPv6, such as built-in encryption and simplified fragmentation, they are not enough that people would continue to lobby for IPv6 after all these years. The address crunch is where it's at.
While I like Dan's concept of a magic moment, I think the above quote asks for too much. There are some easier magic moments for individual kinds of nodes on the computer, and some might well happen before others. Let me focus on two particular kinds of Internet nodes: public web sites and home Internet users.
How close is the magic moment for web sites? Well, web servers can discard their IPv4 addresses just as soon as the bulk of the people connecting to them all have IPv6 connectivity. I do not know how to gather data on that, but as a spot point, I have good networking hardware but cannot personally connect to IPv6 sites. My reason is both mundane and common: I am behind a Linksys NATing router, and that router does not support IPv6. Even if it did, it does not support any sort of tunneling that would allow my local computer to connect to an IPv6-only web server. To the extent people are using plain old Linksys routers, we are a long way away from the magic moment for web servers.
How about for home users? Well, it's the other way around for home users: home users can switch once the majority of public web sites have an IPv6 address. This status is easier to gather data on. I just looked up the top ten web sites (according to Alexa's Top 500 Web Sites) and checked them with a publicly available IPv6 validation site (http://ipv6-test.com/validate.php). Of the top ten web sites, only four can be reached from an IPv6-only client: Google, Facebook, YouTube, and Wikipedia. The other six still require IPv4: Yahoo, Baidu, Live.com, Amazon, QQ.com, and Twitter. As things stand, we are also a long way from when home users can switch to IPv6-only.
Overall, this was a cursory analysis, but I think these "magic moments" are a helpful framework for thinking about the IPv6 changeover. Unfortunately, this framework currently indicates that we are nowhere close.