The lack of SNAT/DNAT targets in Linux ip6tables makes it quite impossible to use ipv6 for any serious enterprise networking. Ipv6 multihoming can't be done without BGP, other solutions like mobile ipv6 or shim6 are - at best - a big mess, also who wants to broadcast his internal network topology/numbering scheme to the whole internet ?
There seems to be some kind of religious taboo here, where the only - supposedly - evil use of NAT (N-to-1 mapping) being taken into consideration, but this is IMHO just plain wrong. Also the NAT haters main argument is that it doesn't preserve end to end reachability (which is not even true for N-to-N mappings), but without NAT everyone is gonna use a stateful firewall for ipv6, and guess what ... the effect on reachability is almost exactly the same.
The other problem I have is with anonymity, without NAT every PC in your local network may be identified individually, there are many cases where this may not be desirable.
IMO ipv6 brings some nice new stuff to the table, the most obvious being the xxl address space, but takes away too much for me to consider using it for myself or my customers at the moment.