The three most important principals for a desktop Linux distribution in my opinion:
Have a stable base (kernel, system libraries, etc)
Have an up-to-date desktop environment
Have up-to-date apps
Other than those three principals, it’s also important that it…
Doesn’t get in your way. OpenSUSE is a big offender in this matter because of YaST. Other offenders could be things like strange SELinux policies that throw you off, a package manager that’s very slow to use (Fedora’s DNF gave me this problem), and awkward defaults.
Has all the software you ever need. Ubuntu and Arch are the best at this. Thanks to PPAs and the AUR.
Makes it easy to upgrade from major version to major version of the distro (For some, you have to re-install the complete distro to upgrade.)
If it’s a beginner-friendly Linux distribution, it’d be nice if some safe firewall rules are put in place. Perhaps this contradicts my “doesn’t get in your way” idea, but I believe a desktop distribution shouldn’t allow incoming network requests by default. This is not a super import principal to me bear in mind.
For these reasons, I now believe Ubuntu (more specifically Kubuntu) is the Linux distribution that meets my needs. I’d probably go with Linux Mint or Elementary (both Ubuntu based) if I wasn’t so sold on the KDE desktop environment.
Through the default Ubuntu software repository, PPAs, Flatpaks, Snaps, and DEBs that certain companies distribute, you’ll pretty much always be able to get the software you need. Flatpaks, Snaps, and PPAs tend to all be bleeding edge, up-to-date software.