Windows -- I'm completely flummoxed by it, never used it, no understanding of it, and at my age, too old to "start over".
Ah, I don't think anybody is ever too old to "start over".

But yeah, Windows does a few things differently, and it'd take some head-scratching to get used to the differences. I don't think it'd be that hard to switch, though.
Linux -- a possibility. Lack of software seems like an issue. Troubleshooting would definitely be an issue.
Lack of commercial software is definitely an issue, although less so than in the past. Some high-end applications are available, many are still not. But if you're willing to use non-commercial software, Linux has it all.
Troubleshooting could be a problem, but perhaps less than you might think. If you go with a very user-friendly distribution (Ubuntu is a good choice), you can minimize the amount of time you need to understand how to use the command-line. Also, OS X really is Unix, so you might be surprised by how much of Linux you already understand. (Particularly if you do any command-line work on OS X.)
One of the most annoying aspects I've found in modern Linux GUIs is that they're slowly giving up on making up their own user interface style, and instead simply stealing concepts from other OSs (particularly OS X). I've been getting acclimated to the current version of Ubuntu, which has:
- A bar across the top of the screen, containing the menu bar of the current app on the left, and the clock and some widgets on the right.
- A dock (across the side or bottom), with icons for your most popular apps on top/left (with a little dot showing which ones are running), and a trash can on the bottom/right.
- Utility applications, such as "System Settings", which when you open it looks identical to "System Preferences" in OS X.
So, you may find that some distributions may take very little effort to learn when coming from an OS X environment.
