Honestly the biggest thing is just READ WHAT IS ON THE SCREEN. So many people just refuse to read when the computer is literally telling them exactly how to resolve a problem

Communication is the key. And the problem is most Linux users aren't able to grasp how the language they use is opaque to new users who don't share their knowledge base. Just the word distro is already a barrier to new users cuz they don't know what it means and yet Linux users throw it around as if everyone knows what it means. These basic terms are the biggest barrier I think. Most people who just use a computer check their email don't know what a bios is. They don't know how to boot from a flash drive. That's going to be your biggest barrier. Language and the basic stuff you don't think of as remarkable.

cd command.

No joke, when I started, it was the thing I stalled on the most as it's so basic no one explains

The way Linux treats many things as part of the file system (devices, sockets, etc.) that Windows doesn’t.

Honestly if you can install windows on a machine and use it then you can install linux on a machine and use it. especially if its an out of the box distro (like my favorite zorin). when downloading something from the web you have to choose linux instead of windows and its usually .debian so its good for it to be a debian based distro. if someone else install the linux for the person then they just need the same skills they needed to run windows. mouse moves the same. login is same. doubleclick is the same. etc. etc.

Rtfm. That problem? it's almost always a permission issue.

How to backup their home directory, even if it is a basic method like mounting an external drive and drag and dropping files.

If it's just Desktop usage, not much difference than using anything else. Make sure to install updates when asked, and that's about it.

Just be clear that downloading anything for Windows will not be compatible.

Something I don't think is mentioned because someone who knows how to use windows can figure it out: what to do when a window freezes, the keyboard command to reboot, other common things that go wrong and how to get around them. I use a gui task manager and it's basically just like windows, but if the computer freezes on someone who doesn't know what to do you will be receiving a phone call.