Init system used to be a big point of deleniation, but I think systemd is the standard now (for better or worse)
The filesystem and networking stack still have some variability.
There's still default applications, kernel modules, a gui app installer, the desktop, included drivers, and many more things that go into a distro. If you go with a distro that uses KDE and you switch to GNOME for example, you might lose a lot of GUI support for customization, might have to build addon packages yourself, etc.
It's all open source at the end of the day, but that optionality leads to a less streamlined user experience and lack of guardrails as soon as you step off the beaten path, than you would get with something like OSX
