According to a few sources playing the F&F Beta, it seems that Blizzard may have found a way to completely remove the delay of character combat actions resulting from network latency. This is further evident in many of the gameplay streams and youtube footage, in which nearly all combat actions appear to be instantaneous as if played in single-player. From what we understand, this only applies to combat actions, and not other gameplay functions, such as looting, chests, salvaging etc. The exception being big lagspikes, which will always cause a delay.
This has been corroborated further by F&F beta players who live in Australia. They are reporting that they are not seeing any delays in their combat actions, while having at least 200ms latency.
F&F sources have it that Blizzard are using a new method of client/server communication that provides a secure and tamper-proof method of using the local client to handle its own calculations, while sending the data to the server for verification. The Client doesn’t require the server to respond, hence removing Latency from the equation. The exact technical information of how this works is unknown, particularly how it protects the game from tampering with the client, however it is believed that the server checks all the data that comes from the client, and if it detects tampering it disconnects the client. This eliminates the need for the client to have to wait for a response from the server, and runs as business as usual unless the server doesn’t like what the client has sent it.
This is huge news, particularly for players in Australia, New Zealand and other high-latency regions that don’t have localized servers. Diablo 3 combat looks very fast-paced and the slightest delay would make the game feel rather broken. As well-illustrated by this monk gameplay video
http://www.youtube.com/watch?v=PJahkoqY-ak