Latencies will go through the roof, and P2P will be harder because of the difficulty in establishing connections without the help of willing 3rd parties with routable addresses (to act as proxies, or provide STUN-like functionality, etc). (Geolocation will probably stay about the same, but could go down depending on the depth of the NAT'ing... For example, if you are behind 2, you will only be seen as the outermost address...)

If it weren't for the latencies and the liability, I would consider a P2P IPv6 overlay an acceptable transition plan.