The user and permissions system. I struggle with things being set up on the root account vs user and vice versa, occasionally breaking my system by editing config files or with CLI commands. Did that a few weeks ago trying to get my PC to read USB-serial without launching the program from CLI with sudo+pw. I had an extra newline in one of the config files; it didn't like that! I cannot follow the decision-making logic that I would need sudo to talk to my embedded device. I've heard the explanations about connected storage devices, but I cannot agree with it. On windows, the same hardware, and same source code (compiled to a different ABI) just works, because its permission system is not so strict.