Not too long ago, HDMI released HDMI 2.0a, which supports only "static metadata" for its High Dynamic Range (HDR) feature. This means that when you are watching a UHD movie with HDR data, it uses one overall HDR grading for the entire movie. With HDMI 2.1, the HDR data, or "metadata," to be exact, can be dynamic.
In other words, the HDR grading can be different from scene to scene. There can be more HDR in one scene, less HDR in another, none in yet another scene, or any combination thereof. This means HDMI 2.1 will be able to do various versions of HDR, including Dolby Vision (based on HDR10 + 2 bit of dynamic metadata), Technicolor and Philips HDR (which are completely dynamic), and even Hybrid Log Gamma, which is a combination of SDR gamma curve (used on the darker side of a scene) and HDR gamma curve (used on the brighter side of a scene) developed by NHK (Japan) and BBC (UK).
They all sound and look fine and dandy and technically interesting to me. And considering what I've experienced in various studio demos, I prefer the HLG (Hybrid Log Gamma) shown to me by NHK Japan. But how will it affect you as a consumer?
For one, you do not need to change your HDMI cables. All of these varying standards still use the same 18Gbps bandwidth required for HDMI 2.0 and 2.0a. So as long as you have a decent cable to begin with (I strongly recommend PXLgen THX-certified cables, which are relatively inexpensive and made by a Canadian company) then you are safe.
But for receivers, this is where it gets tricky. Just like HDMI 2.0 can be upgraded to HDMI 2.,0a via firmware update (my Pioneer Elite SC-95 did that automatically), any HDMI 2.0/2.0a chips can theoretically be upgraded to HDMI 2.1 since they all are utilizing the same chip and bandwidth, just a different protocol. Whether the companies are willing to do so is a completely different story.
I can understand when a TV company refuses to upgrade its HDMI 2.0a to HDMI 2.1 because the display itself should be able to translate the data and display the image accordingly. However, as pre-pro, receivers, UHD source devices are only outputting/throughputting the signal, and there is absolutely no limitation for the upgrade not to happen.
Besides, other than Dolby Vision (which theoretically has been demoed using the regular HDMI 2.0a), I see this upgrade more like the deep-colour upgrade of HDMI 1.3 of yesteryear. Everybody updated the HDMI version, but the Deep Colour content never showed up. Although in this case, I truly hope Hybrid Log Gamma will become the final standard for HDR.