On a Mac, you can cut/copy/paste using predictable keystrokes in any application, anywhere. And there is a single clipboard. On Linux, you have to deal with a multitude of behaviors (middle mouse button, Ctrl-C, others, and does it paste where the cursor is or do you have to click the middle mouse button exactly where you want stuff pasted, etc).
And I'm not even talking about multiple media types: all I need is plaintext.
The second dream wish would be for consistent keybindings in all text input boxes, like in Mac OS. Such as make Ctrl-A always go to the beginning of line, Ctrl-E always to the end, and Ctrl-K always kill whatever is in there.
If I had that, I think I could use Linux without getting annoyed every couple of minutes. I would still miss apps like TextExpander, flawless drag&drop, and lots of other things, but I could at least use it without frustration.
Unfortunately, none of this is likely to happen. If you wonder why, it's enough to look at the responses that will surely land here: