There is only one issue I'm having with macOS or Mac OS X today, they still use 10.x and they had three wonderful possibilities to switch to 11:
Lion: Darwin 11. (Also leaving behind PowerPC) El Capitan: Simply remove the 10. High Sierra: Sync with iOS 11.
But Apple didn't do it.
On a serious note:
The other thing I really disliked was FileVault 1, which based on a sparse image file and was an horror to backup. And in Panther it was at first extremely unreliable. I think I lost some data back then. FV2 based on CoreStorage was much, much better. Which came with Lion btw.