Many new Ultra High Definition (UHD/4K) displays support more than an increase in the number of pixels (resolution). They also can display High Dynamic Range (HDR) material. The chart above shows the difference in what cameras (media) can capture versus what displays can render.
All displays have a limit as to how colorful (saturation) and how bright (lightness) they can go. HDR allows the display to show more of what the camera can capture. This chart from SMPTE shows that the displays are still dramatically more restricted than the cameras, but HDR is a significant step toward representing what the cameras can capture, but are not very close to the full camera range.
HDR can support a wide range of source material beyond HD, but that flexibility brings with it a much higher level of complexity. Displays must recognize the data being sent to it and adapt accordingly. Every device in the video chain along with the source must properly handle this new data type as well. HDMI 2.0a and HDCP 2.2 are required to support these sources properly. Unfortunately, very little UHD/4K devices have been sold to date that meet these requirements.
Sources available also vary in their degree of support of this increased capability. Most of those available are DCI P3, digital cinema masters, while a few will have greater luminance and color saturation ranges. The benefit of greater saturation and luminance capability is also not very evident in all images unlike more pixels which can be seen in every scene.
M-Go has been disturbing a few very high quality HDR sources. The clarity of these images is staggering. I suspect it is not only the result of greater dynamic range, but higher precision from 10 bit sources with little compression. I have seen similar clarity and smoothness from film masters in post production on color grade displays. I suspect much of the improvement visible is from increased precision and less from greater saturation and luminance capability although the increase in luminance range was obvious in certain scenes.
Consumer displays historically have a poor record with respect to reproducing a single color standard properly without expert setup assistance. I see almost no consumer displays that have good Rec. 709 color without significant intervention with sophisticated tools. HDR will face significant challenges in actually obtaining all of the improvements possible unless consumer displays greatly improve their quality control that has historically been present.
It has also been demonstrated in a SMPTE (1) article that the standard for HDR is incorrect with respect to color conversions. This needs to be resolved quickly if HDR is going to meet the expectations people have for this technology.
HDR offers a lot of promise for higher quality images, but the challenges for it achieving market success are not minor.
(1) L. Borg: Better color matching between HD and UHD content, SMPTE, April 2015