The Retina Display was very impressive when it was introduced in 2010, but I believe not so much for the hardware itself, as for the whole idea behind it. The Retina display was really a display with pixels much closer to each other so you could hardly see the space between them. It was Apple’s software that really made it shine : the idea of not having more content on the screen, which would have been tiny, but rather have the exact same elements drawn with 4 times the number of pixels, making them much sharper.
And on the iPhone 4 the sceeen was laminated so it really looked like it was a sheet of paper. One comment that came back often then was that it looked fake, like the sticker that displays a picture of the desktop on a fake device in a shop, it was that good. Of course now it seems completely natural but in 2010, phone displays were much lower resolution than the first Retina Display, and the general idea was still mostly « more pixels = more content ».