Robert Ancell, a Canonical employee and Mir developer, wrote a blog post yesterday entitled "Why the display server doesn't matter." In the personal blog post, Ancell argues that for too many years the X display server has been in use but finally we're reaching two new contenders for next-generation display servers: Mir and Wayland-based compositors. Robert Ancell states, "The result of [applications accessing the display server via a tool-kit and hardware/drivers becoming more generic] is the display server doesn't matter much to applications because we have pretty good toolkits that already hide all this information from us. And it doesn't matter much to drivers as they're providing much the same operations to anything that uses them (i.e. buffer management and passing shaders around)."
Ancell does say though that open drivers do matter as does the shell. Open-source drivers matter to be able to fix them and the shell matters for being able to support multiple form factors.
At the time of writing, that blog post has already yielded 78 comments on both sides of the table from both Mir and Wayland stakeholders, including many colorful comments.
The most visible counter to Robert's blog post is now a personal blog post by the KDE KWin maintainer, Martin Gräßlin, entitled "Why the Display Server DOES matter." In that post Martin says he's shocked that Canonical doesn't see problems in having multiple display servers and argues that there are problems with having to support multiple different display servers. Martin then goes on to explain various issues he has had in porting KDE Frameworks 5 software to Wayland for various portions of code that don't cleanly fit within the toolkit realm.
Martin ended his blog post with, "Canonical created a huge problem by introducing another Display Server and it’s affecting all of us and they are still in denial state. It’s not a simple the toolkit will solve it. It can cause issues everywhere and that affects the development and maintenance costs of all applications. My recommendation to application developers is to never accept patches for this mess Canonical created. If they want that fixed, they should do it in their downstream patches. Distro specific problems need to be fixed in the distros. I certainly will not accept patches for the frameworks and applications I maintain. This is not for political reasons as it’s so often claimed, but for technical, because I cannot test the patches (Mir not available on Debian) and our CI system cannot build it."
Update: KDE's Aaron Seigo also published a blog post siding with Martin on why the display server does matter. He says the display server matters to applications, the desktop shell, tech support, drivers, and hardware vendors.