In the last five years more or less, the way we use "computers" have changed a lot, specially because of Apple's iPhone and iPad, but also thanks to the offers of software as a service in the cloud. Because of this everyday less people depends on Windows for their day to day "computing" work. This may open a possibility for GNU/Linux to triumph.
If we see this opportunity a little bit closer and in more detail, we'll see that there are two main problems in GNU/Linux' way to success.
Some time ago, almost everyone who turned a PC on was to work on one of the components of the Office suite from Microsoft, and to browse the Internet. Out of these two tasks only the MS Office one GNU/Linux can not execute with full performance. Yes, we have OpenOffice and LibreOffice and some other options, but we have to admit that there are compatibility issues, specially if you are dealing with macros in excel.
OpenOffice nor LibreOffice have enough support to defeat Microsoft and set its product away, but some others have it. Google is one of them, and GoogleDocs is more and more used everyday. Even Microsoft had to go to the "cloud" with MS Office.
Another problem is hardware compatibility. I am not going to say that these issues nowadays are greater with GNU/Linux than with Windows, they are more or less the same. If one needs to install Windows will face almost the same problem (if not more) than if he were installing any GNU/Linux distribution. The main difference is that MS Windows comes pre-installed in the majority of computers sold these days.
When you buy a PC or Laptop with Ubuntu pre-installed you have the same experience than with Windows, you only have to turn the PC on and start working (Actually with Ubuntu you have more pre-installed software)
Out of these two problems, we can see that the use of tables and smart phones is making people go more and more to MS Office alternatives as you can't install it on the tablets and smartphones. This situation helps GNU/Linux as it can also use these alternatives, specially the "cloud" ones.
The second problem is not going to be solved by itself, the companies behind some GNU/Linux distributions, will have to work and make efforts to put more and more pre-installed systems available in the market.
I can see that it is Canonical the one with more experience in this field. Canonical is now working hard with hardware manufactures to offer Ubuntu Mobile, and also have some experience delivering laptops with Ubuntu pre-installed.
Canonical is now in a good position to offer a complete set of pre-installed products (laptops, tablets and smartphones).
This will eliminate the need to install the operating system, or configure anything, it will be just a matter of turn the device on and start using it. This is the way the average Joe and Jane want to work with "computers"
A lot of people do not like Canonical and/or Ubuntu, but this company is the one with more opportunities to make one GNU/Linux distribution succeed in the market. To achieve that Canonical is using some of the tools that Free Software purists hate the most. It is taking Ubuntu away of the Free Software philosophy. But if someone wants to see GNU/Linux succeed it will need to come with support for closed hardware, support for closed applications, and will have to make some concessions.
Some companies not so loved in the Free Software community are making this possible, I am taking about Canonical, Apple and Google, maybe without even knowing it, but they are moving the market to a point where the massive use of GNU/Linux or at least one of its distributions may be widely adopted.