[Quoth]

Eye strain, in order:

1. Glare / reflectance from a cheap surface. Micro etched is expensive, more so with glass and especially with sapphire.This is why many ereaders have plastic top layers and Apple products are extra shiny. [1]
   
2. Poor contrast [2]
   
3. too bright or too dim
   
4. Low resolution or smudged text.
   
5. Text too small (easily fixed on most ereaders)
   
6. Need different prescription of reading glasses.

[1][2]a More layers always reduces brightness and contrast. More frontlight on eink only helps brightness and never contrast. The Triton and Kaliedo panels use a colour filter layer. Capacitive touch and frontlights add layers. This is why some Pearl and early Carta with no frontlight and no touch or IR touch are pretty good.

Note: if reading distance / specs prescription really poor that rises to 2. Item 1 will give headaches if the reflections are moving.