The linux landscape is very diverse and complicated... but a simplified description would be:

A linux distro consists of:

1. a kernel. the famous core project led by Torvalds and contributed to by thousands of others. This provides the low level functionality necessary for any computer to run. Contains basic stuff like file system, process management etc.

2. a userland. This is all the stuff on top of a kernel like a desktop environment, applications(text editors, web browser, etc.), various utilities, a shell(often Bash) to alllow the user to communicate with the underlying operating system, a package manager to install new software, an init system (such as the famous systemd) etc.

[Aside. The term "Gnu plus Linux" refers to a linux kernel complimented by the Gnu userland]

When someone refers to a linux distro's base they would usually be referring to the composition of the userland that is included (though sometimes they might be referring to a modified kernel as well). So people who are familiar with the intricacies of the apt package manager (associated with Debian and others) might not like that Centos uses yum (though replacing the word apt with yum works on many commands [ e.g. sudo apt install myprogram].

The desktop enivironment will use a particular file manager by default so that can be a point of friction for someone who swaps distro; menu design might be slightly different, icons in different places.

If someone is used to systemd to start and stop services then relearning a new init system could be enough to stop them switching distro.

A different terminal emulator can be used by different distros but to be honest with you I've never spotted any difference.

Some distros choose different C libraries (musl, glibc) but if you are not a programmer this might never become apparent to you.

Switching from one way of doing things on linux is usually not too difficult if you do it in small chunks but if you change a lot of things at the same time it can definitely be overwhelming.

Also some distros are targetted at niche users. If you accidentally overwrite your os with a new distro and only then find out that there is no gui and networking isn't on by default and you have to edit your own network config to get internet working then you might avoid that distro in the future (Thankfully big distros like Debian, Ubuntu, Centos don't throw users in the deep end like this).

Probably the single biggest hurdle holding back linux adoption is the vast amount of choice which can be very confusing for new users.