How dumb is the client really?

[Kevin]

I have not looked into it, but do have an incident from the early closed beta test (phase 1) to relate.  Most scoffed at what it meant, but do consider the implications for what you are asking even though a lot how the game works has changed since then. 

During that first part of beta the servers were very unstable and you could have some them failing while other servers continued to work fine.  One incident that happened was that basically the world server for environment and mobs basically quit responding to any interaction with the clients, but the chat servers still functioned fine.  There had been very little indication that this happened other than the players around you stopped moving or any other activity.  They just stood where they were when this happened, but the mobs that were in your perception range were still there and continued as normal.  This included my being able to have combat and kill 2 of them before I myself was killed (accidental multi-mob pull and rest of party non-reactive from my point of view).  To me this pointed that the client still has a fair amount of the combat information at its disposal for handling a situation and is most likely only dependent on periodic syncronization signals from the server to stay on track with what really should be happening.  After about 15-30 seconds pretty much everyone that was on was stuck where they at and unable to move; I would guess from the lack of sync signal.  To me it looks probable that a lot of what you want is buried in the client.
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Situations like that still happened after release too. Most vivid memory for me was I was on a flight from X-roads to 1K needles when something like that happened. Treesh was actually still killing stuff and had the same thing you basically describe happen to her. I landed in 1K needles and there were no NPCs and she finished killing her last mob. We were both chatting in guild and general. She was of course unable to loot the mobs (got stuck in wonderful loot lag) They restarted the servers after that one.

Gnolack also explored all of the badlands at L15 when a similar thing happened. No mobs I ran around got stuff mapped, and client showed the exp. There was again another server restart and rollback but I still had half of the badlands explored and the experience for it so some of the feedback was still working. Situation lasted for a good 10-15 minutes, I think the rollback was only about 5 minutes for the characters though.

But yeah I imagine a lot of the info is in the client code. You don't get the ghost towns anymore, but the release servers weren't super stable either. And Blizzard has stated that each server is a cluster so it's very possible for the chat server to be fine and item server to be toasted, etc, and when you are in a situation like that you do see the client can do a lot of prediction including combat and exploration.