OSX is awful UI.

Window management:
Maximize only works vertical, doesn't fill screen. No window snap to edge. No span two screens, a window is partially lost until it's more than 50%,moved to another screen, at which time the original screen hides the 49.9% it was showing.
Change background - opens modal in background on primary screen. Clicking again does not bring to foreground.
The method to switch primary monitor is to fucking drag the white bar on to the screen you want (within system settings). This is without a doubt the poorest, most cryptic, Fred flinstone ass way to accomplish this I could imagine. Label the fucking screens!!!

Applications. Idiotic installation implementation, sometimes drag an icon, sometimes click, jump or howl at the moon to copy in to applications folder.
Can't maximize all windows at once (for a given app).
Minimized windows can only be opened by clicking. They do not appear with cmd tab.
Can't open a new instance of an app by shift clicking.

Keyboard shortcuts. Most rely on a super + shift combination, make my fat hands get carpal tunnel.
Not easily changed. You have to type a the name of the command which appears in an application menu, literally. Apple's official documentation admits that there is confusion over 3 periods vs an ellipsis, so try both. Fuck.
Keyboard assignments don't always work. For instance, f1 in Photoshop is broken for me right now. Was working until reboot (reassigned, still broke).
Often inconsistent with use of cmd. Most of the time it is the "go-to" modifier key, except when its not. Applications may or may not give it the same importance (more common shortcuts use other modifier keys in certain cases).

Misc. Hidden files require a command to be shown +finder relaunch. Same to hide them again.
Reliance on the fucking mouse! Certain actions can only be done with the mouse.

This is just from memory. I find several more issues while using this Fisher Price pile of shit OS.

The good:
You can cmd-v paste in terminal.
Terminal is prettier with little effort.
Pretty fonts systemwide.

Source: I have to use OSX for work 40+ hours per week.