A 2560x1700 resolution screen instead of a 1440x900 resolution screen
FWIW, I've never had any burn in issues with my first gen Retina MacBook in all the years I've owned it. Although based on many of the complaints I've read, that's because I was lucky to get the better screen.
The screen's also 8% sharper. (220dpi vs 239dpi). I don't have them side-by-side right now, so it's hard to say if that part's noticeable. I mostly base my judgement on the quality of the backlight in conjunction with the super-high DPI and touchscreen.