HDR was hugely important to the quality of the presentation. The bottom line is simple - in some scenarios, HDR adds just as much extra detail as the additional resolution. In Horizon Zero Dawn, the skybox is transformed, on Uncharted 4, the beach on the island level we saw is washed out, completely lacking in texture until HDR is enabled. What's clear is that the perception of HDR having an impact on colour vibrancy is only half of the picture. Yes, contrast is massively improved, but in turn, this allows for much more detail to be resolved in the presentation.
The importance of HDR - as impressive as it is in the flesh - will prove challenging to Sony, in the short term at least. First of all, it's a nightmare to market effectively. It's extremely difficult to show the HDR difference via standard media - screenshots and video won't cut it. Secondly, you're categorically missing something special by upgrading to a 4K screen that doesn't support HDR - meaning your choice of display is crucial. Thirdly, the way HDR is presented varies substantially between screens - it can look very, very different moving from one display to the next. The rule of thumb here is to buy a screen with the brightest panel possible.