[Galaxy S 4 Q&A, Help & Troubleshooting] BarryH_GEG:
You need to understand how the human eye works; it's the lowest common denominator. From an earlier 720P vs. 1080P discussion...

The thing people lose sight of (no pun intended) is that the human eye is only capable of processing and resolving images at a certain level. Any detail beyond that level is literally wasted. The 20/20 human eye has a maximum visual acuity of 1 arcminute at 10.5+" away. That is, two points must subtend an angle greater than 1 arcminute in order for a 20/20 eye to resolve the two points. What this means for displays is that pixels must have an angular subtense of 1 arcminute or greater in order for your eye to resolve the detail in the content. Detail below 1 arcminute isn't perceived by the human eye any differently whether a display has 267 PPI or 441 PPI. This is the reason Apple landed on the resolution they did hence the name "retina display." For example, the N2 has an arcminute subtense of 1.073 arcminutes when viewed from 10.5+" away which is nearly perfect and it has a PPI of 267. The DNA has an acrminute subtense of .66 and PPI of 441 which is a waste because anything below "1" can't be resolved by the human eye.​

From Anandtech...

At this size however I have to admit that I find the pursuit of the subpixel geometry more of an educational one than something which affects users. While I could occasionally see it on the SGS3, I definitely do not see it on SGS4. The subpixels are now small enough that whatever the pattern, it all looks like a homogenous light-emitting surface, which was the goal after all. I could bring up the visual acuity discussion again but just trust me that it's small enough to not be visible even with perfect vision. So resolution is great and up to par with all the other LCD-bearing flagships this year, lack of RGB stripe notwithstanding.​

http://www.anandtech.com/show/6914/samsung-galaxy-s-4-review/8

From DisplayMate...

For most uses, however, the extra sharpness of a 1080p display is wasted. In Dr. Soneira's third-generation iPad display shoot-out, he explains that because most adults don't actually have true 20/20 vision—even with glasses or contact lenses—when they view a display further away than recommended, the eyes can no longer fully resolve the sharpness. The iPhone’s Retina display makes a good example of this. "If you use the legal definition for 20/20 vision, the resolution for the iPhone at 326 ppi becomes a ‘retina’ display at 10.5 inches," says Dr. Soneira, because that’s the viewing distance where the image on the screen will appear optimally sharp. After about 15 inches, "the much higher 326 ppi is 'wasted' because the eye can't resolve sharpness above 229 ppi." The further away a person gets, the more gradual the reduction in perceived image sharpness.

When it comes to 1080p on a smartphone, he admits that it might not matter for the most casual users. "For some people, it is possible to tell the difference if we were to sit down and study a [1080p] display and a [720p] display, side-by-side," he said in a phone interview with Ars. "If you’re really a fanatic and you study images, or you have some professional applications and you’re really into displays, then it may make a visual difference for you."

For most people, though, it won't matter. Photos are inherently fuzzy, so it won’t matter whether they’re viewed on a 1920×1080 or 1280×720 smartphone display; you’ll still see their imperfections. "Even the tiniest image detail in a photograph is always spread over more than one pixel," Dr. Soneira explained in a follow-up e-mail. "The image detail is never perfectly aligned with the pixel structure of the display." Videos are even worse: not only are they fuzzy like photographs, but the pictures are constantly moving. Even if the images were sharp, the human brain couldn’t zero in on content that’s appearing for only a fraction of a second on such a small display. "For ordinary viewing of videos, 1920×1080 is really not going to make a visual difference," adds Dr. Soneira.

Where a 1080p smartphone display could really make an impact is with computer-generated content—that is, the user interface, buttons, and text. "Only computer-generated images make full use of the pixel resolution of the display," says Dr. Soneira. "For graphics and text, maybe you want that kind of sharpness."​

http://arstechnica.com/gadgets/2012/10/1080p-on-a-smartphone-screencan-it-possibly-matter/