tl;dr a more powerful operating system with worse UI, which is the obvious trade. A less powerful system with a fantastic UI.
Chances are you're going to think that the thing you've been using for years is "good UI", and anything different is "bad UI". I don't doubt that Apple have spent a lot of time designing their UI/UX to work well for most people, but it feels bad to me because it's not what I'm used to.
Either way, you get used to it. I've found programs that make OS X work in a way that's more familar to me, and learned to work with the things I can't change.
As for the cmd button replacing the ctrl button. Never going back to ctrl+<key> for shortcuts. The command button is so much nicer to use, it's your thumb vs you pinky. You don't need to stretch you pinky/move you hand slightly to have access to shortcuts and that's so nice. VERY desturbing at first (I hated it), but never going back. Or maybe I've just gotten used to using my thumb. Ultimately, I think it makes a lot more sense though. No more alt-F4 to close windows, its cmd+Q, no more ctrl+B to bold a text, it's cmd+b, cmd+h minimizes/hides your window etc etc.
Also, what i found infinitely annoying at first and that I really like now on Mac, is the window switching with the keyboard. On Windows, alt+tab switches between all windows (including different instances of same program). on Mac, alt+tab switches between applications and alt+` switches between different instances of a said program. I find that more elegant when you have a lot of windows opened, which i always have.
But all these are tiny little things that add up to something really nice to use, but each individually don't and will never justify OS X being the best OS in the world blah blah. You can probably achieve a similar behavior on windows/Linux/whatever. What's nice about it though is that it's the default setup, no third party app needed/config files. You install it and it makes sense. That's why I'm not switching from OS X anytime soon, it's ergonomic, the UI is sick, power user features everywhere, and a real unix. I see it as a linux that works perfectly out of the box. I remember the first month I spent on OS X (coming from 4 years of linux, 2 yr Ubuntu and 2 yr Arch, using both full desktop env (KDE mostly) and lightweigth window managers), everything works and works well and everything is ergonomic and makes sense. Put simply, a Mac never (or very rarely) gets in the way of your work. And that makes a big difference.
But then, I just like Macs because of how I use my computer. I get it that some might prefer a Linux or Windows, and I respect that. I'm just extremely thankful OS X answers exactly what I expect from a computer.
You mean like with the keyboard? Like SHIFT+TAB? That's on OS X.
Never found a good replacement in OSX.
