BENEFITS:
-- Costs $0: But that is also the marginal cost of Windows and OS X. When you pay what you must pay for the hardware you want, you can almost always have Windows or OS X at no additional cost. So, if Linux is also available at no additional cost, so what? The benefit to the end user only shows up when building custom hardware, repurposing old hardware, or using a VM--things I care about but the average consumer doesn't.
The zero cost benefit of Linux is of more value to hardware manufacturers than to consumers.
-- Freely modifiable: But you can freely modify Windows or OS X systems, too, with preference settings, or utility apps, or writing your own code in any number of languages, including code that uses system APIs to make big changes.
The fact that Linux lets you change even those things that are not exposed via APIs is once again of more practical value to hardware manufacturers, or certain other businesses or organizations, than to consumers.
COSTS:
-- You have to set it up yourself, and it never seems to work right. The problem is not Linux itself, but the fact that Windows and Mac are pre-installed, pre-configured, and tested by teams of experts, but doing that job for Linux is up to YOU. Few consumers want to deal with debugging system service (audio, network, battery, etc.) problems. Even the little bit they still have to do with Windows or Mac outrages most consumers. The last thing they want is for this problem to be WORSE.
Where Linux has been successful, it has been on the server (no audio, GUI, no battery, etc.), or on client hardware platforms (like TiVo), where the Linux configuration was done by the platform maker, not the consumer. Once again, manufacturers are the direct beneficiaries.
-- You can't run a lot of the software you want to use. This problem has dogged Mac users for decades. It's far worse on Linux.
-- You have to do your own tech support. Your techie neighbors, friends, and relatives can usually help with Windows problems and, in some geographic locations, even Mac problems. But online is all you have for Linux, and some of those people are not as "sociable" as your friends and family. To be a desktop user (or even a Mac user in some places), you have to be a techie yourself or have a close, personal relationship with one. That rules out most people.
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I still want desktop Linux to succeed, too, but I'm not sure what could make that happen. I'm afraid it would take a big, powerful company that DIDN'T already have its own non-Linux OS to push, that would build the hardware and pre-install Linux and somehow resolve the lack of Excel, Photoshop, ....
Google is the only remote possibility that comes to mind with maybe a desktop-Linuxy Android or something, but I'm not holding my breath.