I've spent some time thinking about this and I'd say that for me the features come as groups. For instance, I don't think a vertically staggered board that's not split / angled would be much benefit, but in combination with being split and tented it really helps. Some larger workstation ergonomics are of high importance (chair height / desk height, elbow angle, straight wrists, perhaps arm support, changing positions now and then), but they're not keyboard features.
1. Split and tented (straight wrists with the hands in a natural position) with all the fingers (including thumbs) in natural positions for the home row is the most important to me. This implies vertical stagger and angled thumb clusters.
2. Then I'd say the actual thumb cluster position and number of keys (4 per thumb) with mods on thumb keys.
3. Small total number of keys (possibly with closer vertical spacing) so it's easy to reach all of them without large finger movements and it's easy to use without looking at the board, combined with a good keycap profile or curved plate that provides a curve for the finger when switching rows, with home row caps being lowest and "flattest". This implies using layers to provide all the keycodes, with layer keys on the thumbs. I use 2 on the left thumb in combination (Shift, Fn and Shift+Fn). This also implies a good character layout, with efficient positioning for the user. This will likely vary per person, but I like "alternating" layouts more than "rolling" layouts.
4. Thumb cluster mod positions: Held keys (Ctrl, Alt, Shift, Fn) vs struck keys (Space, Backspace, Tab, Enter). Positioning held keys on the upper, steeper angled part of the thumb cluster allows you to use an inwards gripping motion with the thumb, whereas positioning the struck keys on the lower, flatter angled part of the thumb cluster allows these to be hit with a downwards striking motion of the thumb, like the usual spacebar.
5. Tactile switches with some form of shock reduction to reduce finger shock when bottoming out. Only important when switching from rubber domes, though, as you learn to reduce bottoming out force with tactile mechanical switches fairly quickly.