And NeXTStep itself is to large extent one big ugly hack that stems from experience of trying to build Unix on top of Mach. In fact it is not microkernel, but monolithic kernel running as one big Mach task, thus simply replacing user/kernel split with task/priviledged-task split (which to large extent is also true for OS X).
Correct. The Mach research project at Carnegie Mellon aimed to build a replacement kernel for BSD that supported distributed and parallel computing.
Next's VP of Software Engineering, Avie Tevanian, was one of the Mach project leads. Richard Rashid, who lead the Mach project ended up running Microsoft Research's worldwide operations.
Their work on a virtual memory subsystem got rolled back into BSD.
It did not. Mach is traditional microkernel which provides IPC mechanism, process isolation and (somewhat controversially) memory mapping primitives and not much else.
In late 80's/early 90's there were various projects that attemted to build unix on top of that as true micro kernel architecture with separate servers for each system service. Performance of such design was horrible and there are two things that resulted from that that are still somewhat relevant: running whole BSD/SysV kernel as Mach task, which today means OS X and Tru64 (at the time both systems had same origin as both are implementations of OSF Unix) and just ignoring the problem which is approach taken by GNU/Hurd.
