Does that judgment include QNX? I'm under the impression that it has quite a good performance for a microkernel.
Really, you can do whatever you want in userspace.
This is such a common wish that I wonder why noone has taken up the challenge. Sure, we have Canonical, but Ubuntu is still not at a place where I would say the experience is better than OSX (disclaimer: I use Debian on my primary machine, not an Apple fanboy by any measure)
<rant>Dell had the resources to do just this. They could have put together a team to get a Dell-Specific Linux-based OS and ship it out when they were the top dog. Now they're finding themselves to be increasingly irrelevant</rant>
OK but please keep the good part of X Window System.
How do you display an app launched by one user in another user's session on OS X?
You don't...
How do you remotely log in to one user account on an OS X system while another user is logged in at the physical machine?
You don't...
These (and many others) are trivial (and, very, very convenient) things you can do on a stock (free) Linux (or any other X-based Un*x UI).
I'm not disagreeing with you but simply pointing out that it's not as if Apple got everything correctly with their OS X UI...
How do you remotely log in to one user account on
an OS X system while another user is logged in at the
physical machine?
You don't...
I like that aspect of X too but I find myself getting by without it. I can also SSH in to my Mac as any user regardless of who's logged on physically, which leaves only GUI programs, which I never need remotely.
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.