With SD to HD, the biggest difference to me was the reduction in compression artifacts (many of the initial HD sources were just recycled SD masters, so the difference in detail wasn’t as big as it could have been on a lot of titles, yet they were still distinctly better). From what I’ve seen of 4K (streaming demos), the difference in the demos seems almost entirely due to a better-quality 4K stream–possibly due to better compression, fewer artifacts, etc, but also quite likely due to higher bitrates too. Maybe there’s something perceptible in the resolution itself, but there’s so much apples-to-oranges in the comparisons I’ve seen (e.g. their 2K stream absolutely never has the same bitrate and encoding as their 4K stream) that I’m not sure I’ll ever know if I can see the difference on the resolution alone, under normal viewing circumstances.
I’m always skeptical of technology demos. They are specifically created to look as good as the technology possibly can. How many actual full length releases will stand up to that quality? Only time will tell.
That being said, the HDR demos at CES this year were fairly amazing (as was 8k). And I welcome the advances in compression technology (with reduction in artifacts) as well. I still have a couple of the original recalled Fifth Element BDs sitting around from the early days of Blu-ray.