1) Some weirdness around XCode and XCode Command Line Tools that made installing node-gyp a huge pain. I mean, look at these instructions and tell me if this is something you want to deal with: https://github.com/nodejs/node-gyp/blob/master/macOS_Catalin...
2) Change of default shell in 10.15 from bash to zsh. Kind of minor, but Apple doesn't want to use Bash 5.2 because of licensing. Great.
3) Lack of support for 32 bit programs. This is actually a big deal for me, since a course I am taking requires the use of WinBugs (I know, we should use STAN :) ), which is only available in 32 bit pre-compiled binaries. There is an unfortunately large amount of software like this!