What’s wrong with common keyboards?Firstly, the physical arrangement of the keys makes no sense. Secondly, the logical arrangement (what character is typed on what keypress) makes no sense.
Horizontal staggeringIf you take a look at the common ANSI/ISO layout, the keys are arranged in horizontal rows, slightly shifted left or right. This arrangement is called horizontal staggering, and had an important role in the time of mechanical typewriters, when metal levers had to hit the paper. But this makes no sense today. Our fingers’ natural direction of movement is columnar: we should arrange the keys in columns, not in horizontal rows.
Too much keys - too few layersWith too many keys our hands have to move a lot. We don’t need that much keys at all. Just like we don’t have different keys for capital letters (on most keyboards), we can move some symbols to logical layers accessed by pressing a function key (just like Shift in case of capital letters). This way we could keep the distance of keys from the home row in 1 key unit. Thus, our fingers have to move only to a neighbouring key. Think of how far you have to reach to press Esc, Backspace or any F-key. Even the number row is 2u away. Moving all these distant keys to logical layers under our fingers we improve ergonomy and efficiency of typing.
Lack of thumb keysOn a traditional keyboard our two most powerful fingers, the thumbs, are wasted on a single key: the Space. While the space is the most used key in most languages, you most likely use only one of your thumbs to press it: check the shine on your Space key. While splitting the space is an improvement, each thumb should easily handle three keys arranged in a comfy angle: this is the thumb cluster.
Lack of symmetry and angleOur body is symmetrical, our keyboards should be symmetrical too. If the keyboard is not split, your lower arms and hands are angled inwards (keyboard width smaller than shoulder width). If the board is split, you can use it in shoulder width or narrower - and whatever angle you feel comfortable - the angle is not a problem in this case.
Arrows and numpadReaching over to the arrows or the num pad can hurt our shoulder by itself. Additionally, when reaching over to the mouse, both arrow cluster and num pad are in the way and make hand/arm travel long. Moving the num pad to the left side of the board or using a separete numpad (if needed) may help, but the solution is again: logical layers. Both arrows and numbers need to accessible right below your fingertips without moving your hand.
People are differentThink of how many sizes, types of shoes there are for our feet. And our hands have to cope with practically only one keyboard layout and size. Designing your keyboard around your workplace, your posture, your length of fingers should be just as natural as having more than one size of shoes offered by shops.
QWERTY/QWERTZThe logical layout optimization is a whole other topic. What's wrong with QWERTY? It isn't a disaster, but there are a lot of layouts better than QWERTY/QWERTZ. I mean a LOT. The exact number depends on the language used and the way you evaluate the layout, but it is ca. 5*10^42 with QWERTZ for Hungarian. This figure is probably similar with QWERTY and English. Finding one sympathetic layout from the better ones results in less finger movement, less row changes and jumps, faster rolls etc.