Just a small correction, the Amiga does have pipes, but I don't know how it relates to the thing with the same name on Linux...
Unique thing ? Well probably not unique anymore but the datatypes system rocks completely. Windows still doesn't seem to have anything similar. It's a system that allows programmers to access any data format of certain classes by a uniform interface.
Other than that, the system resource efficiency, very fast boot time and the fact that it doesn't hide itself from the user.
I was gonna say very good multitasking but Linux has had that for ages too IIRC.
