Oh there is lot's Apple should get fixed - also in terms of ease of use...
Things I learned when teaching my parents about macOS and iOS – with the assumption "It just works".
It get's complicated whenever the cloud services come into play. What apps use it, use it optionally, or mandatory, and what does "storage" exactly mean in the cloud and what processes will use how much. Setting up your accounts in "Mail" vs in "Internet Accounts". The frequent password prompts related to cloud services, and how they mix up with OS system password prompts. For beginners it's hard to tell what the OS is currently asking for and why - the prompts are very similar. The iCloud services are fuzzy, hard to grasp and tricky to set up for unexperienced users. If remembering passwords is hard for a user - keychain also isn't very beginner friendly (user password vs keychain password and their odd relation to each other) - macOS should be of much more assistance here than it currently is.
Unrelated to the cloud services, there are still many bad legacy UI decisions lingering around which were introduced with OSX Lion most of all > full screen - Hiding essential controls when going to fullscreen (My mom:"It's frozen, I can't go back, it's stuck") is a no go. Apple making it the default when clicking the green plus icon in windows is one of the worst UI decisions, in recent memory. Full screen modes should be left to the applications, not the OS window manager - for the me worst thing is that the dock doesn't work, I would prefer fullscreen modes to be more lightweight which allows for a quick in and out of this view (currently there is this delay due to a slow animation which feels unnecessary). And who is using dashboard widgets these days? Could have been removed with the notification center sidebar introduction.
Oh and try explaining the good old "Can't empty your trash..." bug still being a thing ... it's decades old by now and never elegantly resolved by Apple > Just show the files in question and ask users if they should be force deleted or kept, while all other files get deleted.