I think the biggest difference between Plan 9 and Unix is that Plan 9 rids systems programming of many special cases by reusing the filesystem metaphor as far as possible. This is a deep and pervasive change, and makes doing most things on the system just a matter of reading and writing to files. This leads to neat things like the /env filesystem, which is a per-process view of the contents of shell variables as files. (This makes it easy to write make (mk) rules that depend on shell variables, just by listing the files/variables as prerequisites.)
Inferno's biggest difference is that it runs the entire OS on a virtual machine called emu. Kind of like Java, but not just a language. (Inferno has a language too, called Limbo, which is closer to Go than C.)
Well, probably not surprising given Rob Pike's involvement in both Limbo and Go.