When doing photography work, you can look at image thumbnails and actually judge sharpness and colour directly off them. And when filling the screen, you can have a much better idea of how they will look in print.

As for text, I've been able to use tiny font sizes and increase the information density since hi-DPI screens— my eyes are good for it. Ahh, iPhone 4 and retina MBP... they were astounding tech at that time.