Additional thumb keys.

The Microsoft keyboard is good (their best yet, I think), but only Space and the two Alt keys can be pressed with the thumb.

On my keyboard², I have Alt, Backspace, Ctrl, and "Fn" for my left thumb, and "Fn", Enter/Ctrl¹, Space, Alt with my right thumb. Slightly less comfortably, I also have Esc (left) and Delete (right).

The Kinesis Advantage 2³ is the most common mass-produced keyboard with thumb keys like this.

¹ It's enter if I tap it, and Ctrl if I press it in combination with another key

² http://www.keyboard-layout-editor.com/#/gists/501437fce9123e...

³ https://www.kinesis-ergo.com/shop/advantage2/

Kudos for Microsoft for that keyboard. Compared to the previous 4000 model they made the 2019 model more ortholinear. Notice the offset between the A key and the Z key. It is not pure ortholinear but it makes possible to type with your pinky Q, A, Z letters without stretching that much. This is the first popular keyboard which goes into the right direction and breaks the 150 years old design!

If your are more demanding then it is becoming not ortholinear enough. Later on you will want to have staggered columns like Keyboardio model 01, Kyria, Kinesis Advantage all have. After you'll try the QMK firmware you'll start to hate that Office and Emoji keys, which are under your strongest finger but are basically useless.

On a split keyboard with two halves, you can independently move the two halves around. This is great if you don't like the Microsoft-designed angle or want to change the positioning.

Other than that, probably just customizability in terms of switches, keycaps, etc.

Customisable firmware. e.g. simulating different "layers" of keys to reduce finger movement, dual-function keys depending on whether the key is tapped or held (which allows for using modifier keys on the home row, reducing finger movement), etc. This can be done with software like kmonad; but I imagine that's less convenient than having it on the keyboard itself.

With the keyboard being physically split, your hands don't have to be so close together.

Also the row-staggering of more common off-the-shelf keyboards just feels so weird after seeing keys arranged in a grid, or vertically staggered.

Honestly not a whole lot. I have the much more expensive ergodox ez and the main features would be mechanical switches, full customization of the keyboard layout and the fact that you can spread them further out. On the ergodox you can have your arms pointing forwards rather than angled inwards. If none of that sounds interesting than there is no reason to get anything more.

Mechanical keys. It feels very great to type with them.