But Chris! I don't care about Unity. What does Mir mean for me?
The two common concerns I've seen on my G+ comment stream are:
What does this mean for GNOME/Kubuntu/etc?
The short answer, for the short-to-mid-term is: not much.
We'll still be keeping X available and maintained. Even after we shove a Mir system-compositor underneath it there will still be an X server available for sessions.
Let's review the architecture diagram from MirSpec:
You can replace any of the Mir boxes which contain the sessions - “Unity Next” - with an X server + everything you'd normally run on the X server. The top “System Compositor/Shell” box is just there to support the user sessions - to handle the transitions startup splash → greeter, greeter → user session, user session → user session, and user session → shutdown splash. We'll probably also use the system compositor to provide a proper secure authentication mechanism, too.
Note that the “Unity Next” boxes will also be running an X server, but in this case it will be a “rootless” X server. Basically, that means that it doesn't have a desktop window, just free standing application windows. This will be how we support legacy X11 apps in a Unity Next session. This is also how Wayland compositors, such as Weston, handle X11 compatibility - and, for that matter, how X11 works on OSX.
The rootless X server in Unity Next will not be able to run KWin or Mutter or whatever. This isn't losing anything, though - you can't currently run KWin or GNOME Shell in Unity, or visa versa, even though everything runs on X.
So, if the short-term impact is “approximately none”, how about the long-term impact?
This somewhat depends on what other projects do. Remember how we can run full X servers under the System Compositor? We can do the same with Weston. Weston has multiple backends; you can already run Weston-on-Wayland, just as you can run Weston-on-X11. Once we've got input sorted out in Mir it'll probably be a fun weekend hack to add a Mir backend to Weston, and have Weston-on-Mir. You're welcome to do that if you get to it before me, gentle reader ☺.
So what happens if KDE or GNOME build a Wayland compositor? Well, we don't really know, because they haven't finished yet¹. Neither of them are using Weston, so a Weston-on-Mir backend won't be terribly useful. If their compositor architecture allows multiple backends then writing a Mir backend for them shouldn't be terrible; in that case, we can replace any of the “Unity Next” sessions in the diagram with “GNOME Shell Next” or “KDE Next”, collect our winnings, and drive off into the sunset in our shiny new Ferrari.
The result that requires the most work is if KDE or GNOME additionally build a system compositor. This seems quite likely, as a system compositor makes a bunch of knarly problems go away. In that case you'd not only need to write a Mir backend for their compositor, you'd also need to implement whatever interfaces they use on their system compositor.
We see this a bit already, actually, with GNOME Shell and GDM; Shell uses interfaces which LightDM doesn't provide but GDM does, so things break.
The worst-case scenario here is that you need GNOME's stack to run GNOME, KDE's stack to run KDE, and Unity's stack to run Unity, and you need to select between them at boot rather than at the greeter.
So, in summary: Mir doesn't break GNOME/KDE/XFCE now. These projects may change in a way that's incompatible with Mir in future, but that's (a) in the future and (b) solvable.
Does this fragment the Linux graphics driver space?
Mir only exists because of all the work Wayland devs have done around the Linux graphics stack. It uses exactly the same stack that Weston's drm backend does².
The XMir drivers are basically the same as the XWayland drivers - they're stock xf86-video-intel, xf86-video-ati, xf86-video-nouveau with a small patch. They're not the same, but they're not hugely different.
The Mir EGL platform looks almost exactly the same as the Wayland EGL platform. Which looks almost exactly the same as the GBM EGL platform, which looks almost exactly the same as the X11 EGL platform. Code reuse and interfaces are awesome - we might be submitting patches to share even more code here.
Weston currently doesn't have any proprietary driver support; similarly, neither does Mir.
We're talking with NVIDIA and AMD to get support for running Mir on their proprietary drivers, and providing an interface for proprietary drivers in general. It's early days; we don't have anything concrete; and even if we did, I probably wouldn't be able to divulge it. But it's likely that whatever we come up with to support Mir on NVIDIA will also support Wayland on NVIDIA.
So, driver divergence - real, but probably overblown.
¹: There's an old GNOME Shell branch that's seeing some love recently, and KDE has a kwin branch for wayland support.
²: On the desktop. On Android, it uses the Android stack.