- Confusion between the window receiving mouse events and the window receiving keyboard events. I'm a programmer, I know exactly how this works, and it STILL startles me on a daily basis when I've been reading text in one window for a minute or two, scrolling around, and then when I start typing, I realize my keystrokes aren't going to the program that I've been interacting with for the last few minutes. What was the last window I actually clicked on? Did I just type random garbage into a code file? An email? A chat window? I have to go find it and fix it before I continue what I'm doing.

- Awkward switching between screens. Waiting for the animation reminds me of those spinning scene transitions in the old Batman TV show [0]. At one point I learned to set the screen animation to be almost instant — there was still an annoying flicker — but the hack stopped working, I looked up another hack, then I got a new Macbook Pro and the notes I had didn't work, and realized I should just stop fighting it. I guess the thing about a "snappy" transition is that you don't see what's happening and could get confused. Makes sense, but very frustrating if you know what you're doing. Now I just amuse myself by mentally playing the Batman transition sound when I switch between workspaces.

Funny thing: early film editors initially thought that cuts needed to be extremely gentle, with explicit cues to help viewers understand that one scene had ended and a new scene was starting. They didn't stick with those techniques for long; it turns out that people are smarter than they anticipated, and they get bored with filler that doesn't move the movie forward! Even a fraction of a second of boilerplate filler to show that a cut has happened (as opposed to moments that serve an artistic purpose) decreases engagement.

- Awkward switching between program windows. If a program has multiple windows, and there's one I really don't care about right now, like an email or document I was working on a few days ago, I don't want to see it randomly popping to the foreground just because I switched to an email or document that I've been working on today. OS X occasionally gets really pushy with me like OH YOU SHOULD REALLY FINISH THIS EMAIL TO YOUR MOM or I HAVEN'T SEEN YOU WORKING ON THIS APP LATELY. Shut up, OS X, I know what window I asked for, and that wasn't it.

- No way to avoid collisions between my keyboard shortcuts and application shortcuts. It's really easy in Linux, thanks somewhat ironically to that Windows key that I cursed when it first appeared. Turns out it's perfect: it's a key that you can bind to window manager shortcuts in full confidence that it will never collide with application keyboard shortcuts. It's a logical division that worked really well with my brain: my window operations and system-level shortcuts all used the same modifier key, and application shortcuts used different modifier keys. You'd think Apple would have something equally elegant, but they don't. You set up key bindings, get attached to them, and then the next program you learn might require you to redo them (or customize application shortcuts to something that makes less sense than the defaults.)

- Menu bar at the top of the screen. I admit this is cliche and somewhat arbitrary, but I was a years-long Apple user, using Apple GUI operating systems all the way back to the Mac Plus and Apple IIgs days, and the instant I saw that Windows programs had menu bars attached to their windows, I realized that's the way it should be. I'm sure other people have the opposite reaction, but I'm an Apple "native speaker" who used Apple GUIs for many years before I saw a Windows machine, and who has used Apple on the desktop almost exclusively for the last six years, yet the menu bar thing still seems weird to me. Why is the menu bar for my program so far away? From a productivity standpoint, on Windows or Linux, if I can see program window, I can visually target the menu. On a Mac, first I have to click the program window, then I can visually target the menu and click on it. It's a small annoyance, but the small annoyances add up.

I'm sure I could go on, but I shouldn't invest more of my morning in complaining :-(

[0] https://www.youtube.com/watch?v=ksuPdtwHCE4