content: Routers, mostly. They tend to have long, long lives and many didn't support IPv6 until recently, especially consumer models and if just one router in the way doesn't support the protocol, you can't use it(though you're almost guaranteed a connection nowadays if both sides support IPv6, there are probably plenty of routes unavailable to you if you use it). NAT solves the problem very, very well, so nobody's been in any great hurry to switch and it'll be put off longer yet if organizations that have been sitting on their leviathan allocations from the early days sell them off.