adamhm: Ubuntu (including its derivatives such as Mint) is the de-facto standard distro, at least when it comes to gaming. Ubuntu & Ubuntu-based distros have the largest share of users and are most widely supported by developers, plus they're officially supported by both Steam and GOG (Valve also recommend that developers target Ubuntu if planning SteamOS support).
And that's why I use them. Still it would be better to just formalize that Ubuntu is the official linux distro so that a normal user will be able to choose the better supported product without getting lost in the "sea of distros".
For a normal user is already difficult to change the default OS.
Power user will still be able to change if they want.
Also for a developer it's different to support one distro (the most used) or to support the official distro.
Finally right now there are incompatibilities between the various distros and between desktop environments, with a official Linux distro it would be easy to know what choices will be pursued for the future and what are only experiments of unofficial distros.
Software on Linux have not at all a common design pattern. There are no clear guidelines on how to be design interface on Linux (everyone uses the library they like). Even this can be confusing and makes the OS a mess.
Something like this:
https://en.wikipedia.org/wiki/Heuristic_evaluation is not possible on Linux. But it is the most important thing in designing a modern application. (just look at Gimp, that nobody want to use just because the GUI is not good and intuitive at all).
It seems silly, but the reason why Linux has problems to establish itself even if Free is its super-freedom (there is nothing official).
In fact, even if Windows is still the most popular system, many have tried linux but then turned back because some problems (due in most cases just to the confusing and not standard graphical interface).
And, despite the mac are very expensive, the spread of mac os remains higher than that of Linux (always for the same reason). Even some programmers prefer mac os since there is a Unix-terminal like on Linux.
For me it's not a problem. I can switch OS anytime, and if I need to use a software exclusive to a system (usually games) I use wine or a virtual machine.
Sometimes I even try to challenge myself with some strange OS like FreeBSD...
But not everyone can do that.
There are many successful open source program:
Firefox, VLC, Blender etc...
There are successful because they are good for people that use them (Blender is not easy to use, but since it is a professional application it's ok to be not as easy as VLC).