There are two keys on standard keyboards for which I have no use: Caps Lock and Insert. Both are intended to bring a slight convenience to a rare operation (typing in all caps or replacing fixed-length data), and both break keyboard input by entering a nonobvious mode where keys do not do what you expect. This wouldn't be a problem except that both keys are located next to frequently used ones, so it's easy to hit them by accident.
Well, not always by accident. I've remapped Caps Lock to Control on every machine I use regularly. This works great, except when I try to use someone else's machine. Then I routinely find myself typing something like C-d to delete a character, and being confused when I find the character still there, and subsequent typing in all caps. This is especially embarrassing when I do it on Windows, where C-d wouldn't have worked even if I'd used the right key.
I've also tried swapping parentheses with square brackets in my emacs (using
keyboard-translate), but this is even more confusing, because it doesn't apply in other software on the same machine, let alone on others. (I've been using it for weeks and haven't gotten used to it.) And even when it works, its benefit is questionable. The square-bracket keys are far enough from the home keys that they're not terribly easy to type - maybe no easier than the regular parentheses. A number of Lispers recommend this swap, but I haven't found it helpful, even for Lisp.
I haven't remapped Insert, because it's not in as valuable a place as Caps Lock, and my home keyboard doesn't even have it. Mac keyboards use it for Help instead, which is only slightly less annoying when you hit it by mistake, and still not frequently used enough to deserve a key. Maybe it's comforting to beginners, but I suspect beginners would find an Undo key much more comforting, as long as it worked reliably. There must be many other common operations worthy of a key (although I can't think of any right now; someone help me). So do we have to waste hardware on annoyances like Caps Lock and Insert?