Multi-user computers are far less important now than they used to be. With systems that compose clouds, you might want the linux kernel and IPC but you don't care about unix permissions models.

I'd like to work on a totally open system where users could clone one another's blocks of memory and see everything. You could come up with a more collaborative environment than we have in unix atm. However, I wouldn't want an arrangement where one person's slip of the fingers could destroy the environment.