IMO the major thing holding back IPv6 on the web is amazon. A huge proportion of services are hosted on AWS, and the lack of IPv6 addressing of instances cannot be forgiven.
I'd say ISPs to start with. In the UK, BT still hasn't rolled out IPv6. Neither has vodafone. Also I have servers in colocation with two datacenters, and for both I had to ask to the support to get an IPv6 range. We are very far from IPv6 becoming a standard feature.
[0] https://www.google.com/intl/en/ipv6/statistics.html#tab=per-...