I vote for:

  - Lack of OEM support
  - Uncertainty of driver support whenever you buy anything
  - Unavailability of some types of software

If usability was the limiting factor why would anyone be using Windows anymore? The hassle with windows software/driver installation and updates alone is a nightmare if you aren't used to it.

When something breaks on Linux you sometimes need to open a terminal and paste in text from support forum. When something breaks on Windows you sometimes need to follow pictures of dialogs or regedit instructions from some support forum. The difference is that you cannot take your Linux computer to any shop and tell them to fix it (or ask your neighbors kids to fix it).

I have no experience with OS X but I presume that it's a mix of both worlds.