1. writing movement-to-pixel latency (should be around 5ms)
2. writing experience (tilting support, pressure levels, anti-aliasing, ...)
3. speed of apps on the device
4. shareability of annotations
5. "stroke-recognition" of diagrams, text, math formula's, etc
6. mashing of touch & pencil gestures
7. apps on the platform
I'd argue that only the Surface 4 solves (1) & (2) completely, with the competition between 75% and 90%. Speed (3) is a solved problem for mobile CPU/GPU/LED/OLED hardware, but e-paper is not close to 50%. (4) is also still quite young, since the "write on digital paper" thing is not even being used by enthusiasts (only some rarae niches show consistent use). (5) is mostly solved for handwriting recognition, and good progress is made on diagrams and math formula's. But it's young, and certainly not yet a commodity. (6) seems quite solved, as most algorithms to distinct between pencils and hand/finger touches work quite well. (7) is not yet solved, there are no good apps. Some exist, but they have "handwriting" as a feature, but not as a core feature.
Color may come in as 8th, but is for most digital authoring not that important. It would certainly increase the potential market for a good e-paper device, but would come last on above list.
In the past 15 years, progress on e-paper display technology has been slow but steady. Is the latency problem solved yet? Or are we still around 100ms?