What adventures like Debian on FreeBSD (kFreeBSD) and even Android teach us, though, is that you can take a great kernel with its sweeping hardware support and build something better on top of it. The hardware work has been done for you. Apple could have easily built OS X and Darwin on top of Linux-the-kernel instead of Mach (what an interesting world that would be), but they had a limited range of hardware to target and the resources at their disposal to tailor Mach to their known hardware targets.
The kernel for the Intel architecture is, I think, at this point mostly figured out. Microkernel had its shot and on Intel we have seen that it's too slow (even though it makes more sense on paper). Linux is pretty darned good, and I think any attempt to be better will, after a few years, end up looking like Linux from several years ago. The Intel architecture has been stable for a long time, and a lot of clever minds have come up with ways to squeeze every drop out of the machine in Linux.
When people say "I want a new operating system!" I'm not hearing them desire a complete kernel rewrite; that's doomed to fail for a plethora of reasons including hardware drivers. I'm hearing them desire a new userspace and GUI. Make something like OS X based on Linux and I'd spend $500+ to buy it and fund its development.