Speaking of readability, I cannot stand these 200 weight thin fonts that only look good on a "retina" device places choose to use sometimes
I was once complaining to a friend how ugly site X is and he said it looked right to him; later when I saw his machine it has a nice display that made site X look twice as better on my shitty display.
Lots of sites are designed on (and for?) high density displays and it ends up looking awful on low resolution screens like mine.
Also most 'normal' users will never pay the x4-x10 premium price for some fancy IPS display and calibrate its colors.