Wayland, Mir, and X - different projects
Apart from the architectural differences between them, which I've covered previously, Mir and Wayland also have quite different project goals. Since a number of people seem to be confused as to what Wayland actually is - and that's not unreasonable, because it's a bit complicated - I'll give a run-down as to what all these various projects are and aim to do, throwing in X11 as a reference point.
X11, and X.org
Everyone's familiar with their friendly neighbourhood X server. This is what we've currently got as the default desktop Linux display server. For the purposes of this blog post, X consists of:
The X11 protocol
You're all familiar with the X11 protocol, right? This gentle beast specifies how to talk to an X server - both the binary format of the messages you'll send and receive, and what you can expect the server to do with any given message (the semantics). There are also plenty of protocol extensions; new messages to make the server do new things, like handle more than one monitor in a non-stupid way.
The X11 client libraries
No-one actually fiddles with X by sending raw binary data down a socket; they use the client libraries - the modern XCB, or the boring old Xlib (also known as libX11.so.6). They do the boring work of throwing binary data at the server and present the client with a more civilised view of the X server, one where you can just XOpenDisplay(NULL) and start doing stuff.
Actually, the above is a bit of a lie. Almost all the time people don't even use XCB or Xlib, they use toolkits such as GTK+ or Qt, and they use XLib or XCB.
The Xorg server
This would be the bit most obviously associated with X - the one, the only, the X.org X server! This is the actual /usr/bin/X display server we all know and love. Although there are other implementations of X11, this is all you'll ever see on the free desktop. Or on OS X, for that matter.
So that's our baseline stack - a protocol, one or more client libraries, a display server implementation. How about Wayland and Mir?
The Wayland protocolThe Wayland protocol is, like the X11 protocol, a definition of the binary data you can expect to send and receive over a Wayland socket, and the semantics associated with those binary bits. This is handled a bit differently to X11; the protocol is specified in XML, which is processed by a scanner and turned into C code. There is a binary protocol, and you can technically¹ implement that protocol without using the wayland-scanner-generated code, but it's not what you're expected to do.
Also different from X11 is that everything's treated as an extension - you deal with all the interfaces in the core protocol the same way as you deal with any extensions you create. And you create a lot of extensions - for example, the core protocol doesn't have any buffer-passing mechanism other than SHM, so there's an extension for drm buffers in Mesa. The Weston reference compositor also has a bunch of extensions, both for ad-hoc things like the compositor<->desktop-shell interface, and for things like XWayland.
Or libwayland. A bit like XCB and Xlib, this is basically just an IPC library. Unlike XCB and Xlib it can also used by a Wayland server for server→client communication. Also unlike XCB and Xlib, it's programmatically generated from the protocol definition. It's quite a nice IPC library, really. Like XCB and Xlib, you're not really expected to use this, anyway; you're expected to use a toolkit like Qt or GTK+, and EGL + your choice of Khronos drawing API if you want to be funky.
There's also a library for reading X cursor themes in there.
The Wayland server?
This is where it diverges; there is no Wayland server in the sense that there's an Xorg server. There's Weston, the reference compositor, but that's strictly intended as a testbed to ensure the protocol works.
Desktop environments are expected to write their own Wayland server, using the protocol and client libraries.
The Mir protocol?
We kinda have an explicit IPC protocol, but not really. We don't intend to support re-implementations of the Mir client libraries, and will make no effort to not break them if someone tries. We're using Google Protobuf for our IPC format, by the way.
The Mir client libraries
What toolkit writers use; it's even called mir_toolkit. Again, you're probably not going to use this directly; you're going to use a toolkit like GTK+ or Qt, and like Wayland if you want to draw directly you'll be using EGL + GL/GLES/OpenVG.
The Mir server?
Kind of. Where the Wayland libraries are all about IPC, Mir is about producing a library to do the drudge work of a display-server-compositor-thing, so in this way it's more like Xorg than Wayland. In fact, it's a bit more specific - Mir is about creating a library to make the most awesome Unity display-server-compositor-thingy. We're not aiming to satisfy anyone's requirements but our own. That said, our requirements aren't likely to be hugely specific, so Mir will likely be generally useful.
To some extent this is why GNOME and KDE aren't amazingly enthused about Mir. They already have a fair bit of work invested in their own Wayland compositors, so a library to build display-server-compositor-thingies on isn't hugely valuable to them. Right now.
Perhaps we'll become so awesome that it'll make sense for GNOME or KDE to rebase their compositors on Mir, but that's a long way away.
¹: Last time I saw anyone try this on #wayland there were problems around the interaction with the Mesa EGL platform which meant you couldn't really implement the protocol without using the C existing library. I'm not sure if that got resolved.