Mach is a microkernel. That means it's a way to design an OS such that most of the things application programs rely on are outside the kernel, provided by servers that can be restarted and replaced without rebooting the whole system. For example, the majority of the filesystem is provided by a filesystem server, most of the networking code is in a networking server, and so on.

You begin to see why a microkernel that needs to be bounced more often than, say, Linux, which is not a microkernel, begins to lose some of the appeal of having a microkernel in the first place.

(Aside: Mac OS X is based on Mach, but practically all of the stuff applications see is provided by a single server which (to the best of my knowledge) can't be restarted without taking down the whole system. It's a hybrid design.)

Also, just to confirm a stereotype, do you use Windows for your servers?