Would be the concept of adhering to standards. An OS that reinvents itself drastically with every new version requiring many drivers to be rewritten is hardly the most efficeint way to go. Linux by being built on what has gone before means that once you have learned a way of accomplishing a task it will most probably still work whwn you upgrde to next version.
When you are using a GUI it doesn't require you to be an aero-space enginneer to point with your mouse and click but to some, who contribute to this forum, pointing and clicking in linux is a world away from pointing and clicking in windows, even though some of them work in IT.
A good test is when an averge windows user, typically has not a clue regarding set up email accounts, network connection - set up etc, uses a linux desktop for the first time.They will be opening aplications, browsing the web, sending and receiving email once they know what menus and icons to click, with the added advantage of it being much harder to screw up the system than with a windows system.
For the more advanced user linux gives you a system that will do what you want it to do, the more you are willing to learn the more power and flexability you will have. With the latest tools you can perform configeration and admin functions with the aid of a GUI or with a command line and knowledge, a very fast and efficeint way of handling these tasks.
The main reason the "linux is hard" myth has been perpetuated is narrow mindedness and a fear of learning anthing. Linux can be almost the same to use as windows and also comletely different but once you take the plunge and give it a fair go, with an open mind, you will find how easy it can be.
Qf course, with a disscusion topic such as this I'm sure we will see a few of the "fan boys" from both sides chucking in their 2 cents worth, the popularity argument, the lack of gaming etc . These are all applcations, not the OS itself. After all if Microsoft wanted to, they could release a linux version of MS Office if they so desired.
I myself don't care if linux never becomes a dominant or major player in the desktop computing world because I will choose to use it untill something better comes along with the same ease of use and value for money and linux will continue to be the OS for the serous jobs, servers, power users,scientific research, and fo other "industrial strength" computing