You absolutely can tell the difference. The 3k display on my 15" retina Macbook Pro looks fantastic compared to a traditional 1080p monitor. You may not notice in a movie, but you can absolutely tell when reading text. Now stretch that same image onto a 27" display, and a 5k resolution is about right.
Good thing a 27" iMac isn't a traditional 1080p monitor then, eh?
The 27" iMac is 2560x1440, and is retina at about 32" viewing distance.
ie, at 32" away from the screen it is physically impossible for the human eye (with 20/20 vision) to distinguish individual pixels due to the laws of physics.
As it happens, most people's viewing distance is slightly closer than that, thus making the display *almost* retina already, but not quite.
I have measured before and I sit about 26-27" away from my screen, so under the line. Even so, the difference is not "night and day" as some people claim between my 27" iMac and my 13" rMBP.
At typical viewing distances from each screen, they look the same to me - i.e., text looks sharp and crisp, figures look well defined, UI elements look sharp.
If anything, the retina screen looks worse for certain applications because they are not high-DPI aware - like ChemDraw, which is still not retina-aware in 2014, meaning the UI and the images and graphics in it look blurry on the 13" rMBP, but not on the 27" iMac. This is a specific problem relating to the software, however, and not the hardware.