[azernik]

The big one is the address space expansion, which in addition to solving "can't get addresses" also makes routing cheaper and faster (can allocate addresses based on network topology, rather than on what's available). Whole they were breaking backwards compatibility anyway, they also introduced a faster way of getting initial addresses, changed headers and dropped some features to make routing easier to implement in hardware, and added some features to make annotation of packets in the network backbone during transit easier.