The most important thing to consider is that we should not try to just copy our iPv4 based concepts onto an IPv6 network. Creating an IPv6 network is more than just doing the same we have done in IPv4 with IPv6. The introduction of IPv6 offers the opportunity to create new concepts for network, addressing, security, management by using all our experience from operating IPv4 networks, but adding the advanced options and flexibiity of IPv6.
As for instance our conservative thinking about address space is deeply ingrained in our cells, it takes some time and more than one shot to create and adequate IPv6 address concept, which should be freed from conservative address space thinking.
The importance of this is in the fact that we have the opportunity to create new concepts and we will live with these concepts for probably some decades. So it is worth to take time and do it carefully.
If you want a list of technical things that go wrong, this is a question too complex to answer and it really depends on the enrvironment and the requirements of your network. Also there isn't too much enterprise migration experience out there yet, as we are still in an early stage of deployment..
You may want to become a member of http://gogo6.com/ a fast growing community for people that want to exchange knowledge and experience about IPv6 deployment.
Hope this helps, Silvia