To my mind (and preferences of course!), the standard IBM PC layouts have never quite
got it right.
First, the original 83-key IBM PC keyboard from 1981:
The numeric pad is too squashed up against the main area, and even seems to flow around it (notice how the zero key overlaps). The return key is rather narrow and feels as though it's quite a way from the main typing area. That right-hand side of the keyboard just has a generally cluttered and awkward feel to it.
The other big criticism of this layout is the small left-hand shift key and the backslash squeezed in between that and the Z. Why somebody at IBM came up with that idea, I'll never understand.
On the plus side, the Ctrl and Esc keys are both in the correct places.
Not really related to the layout as such, but these keyboards also had no caps/num/scroll lock indicators, and as these toggles are carried out in firmware on the PC, you have no way of telling what state they're in before hitting a key.
Next, the 84-key keyboard launched with the PC-AT around 1983/84:
Much better overall layout, less cluttered with the numeric pad set off from the main area. The backslash key has been relocated to a more sensible place leaving a much larger shift key next to Z, where it belongs (but see later). The return key is also vastly improved, and the keyboard gained the caps/num/scroll lock indicators (not visible in this picture).
However, the one big mistake with this layout was that Esc was relocated to the numeric pad over on the right! (With backward quote/tilde in its place to the left of the 1 key.) The Escape key belongs top left in my book.
On to the 101-key "enhanced" layout:
Escape has been moved back to the top left, but unfortunately it's now on its own in line with the relocated function keys instead of being in the main group. Better by far than where it was on the 84-key layout, but not ideal.
The Ctrl key has moved down below shift to make way for caps lock. As somebody who grew up with a Ctrl key pretty much always being at the left of the middle row of letters, I've never liked this. (In fact I have a little TSR utility which hooks into the keyboard routines and changes Caps Lock to Ctrl.)
The return key shrunk on this design to make way for a bigger backspace key and a inexplicably large backslash key. A revision which soon became popular on the clone keyboards was to leave the backspace key at standard size, keep backslash next to it, thus leaving room for a large L-shaped return key like on the previous AT design. Personally, I think that's a definite improvement.
All of the above relates to the standard U.S. versions of the keyboard. Unfortunately, on U.K. versions the backslash key is still
located between left shift and Z to this day. Given the complaints that IBM received about this 25 years ago, it's a mystery why the American keyboards abandoned it, but not the British. I use a standard U.S. layout at home, and whenever I have to do anything on one of my outside contracts and use their systems I'm forever hitting \ instead of shift.
The U.K. enhanced keyboards also have ¬£ on the 3 key, and then have an extra key added to the right of the apostrophe for #. I hate that position, as I'm used to # being up on the top row.
The extra key on the middle row also means that the return key on U.K. enhanced keyboards is a little smaller and less L-shaped to make room, although that's not really a problem in itself.
For some inexplicable reason, U.K. PC keyboards also switch the position of double quotes and @. Never have been able to figure out why.