The differences when you get to 12ft viewing distance, which I would guess most people would be sitting from the screen, even in smaller UK homes, becomes less obvious. However they are still there.
I have had people in a room looking at the same brand panels but different resolutions, and all have been able to tell the difference, eve those that have said there is no difference from that sort of distance.
I have gone from a 500M 1080 Kuro back to a PHD8 Panasonic which is only 768 as I prefer the overall image from the Panny, it looks sharper, however, there is slightly more detail on the Pioneer 1080p panel as there was on the 1080p Panasonic I had before the Pioneer, but overall there differences are subtle.
If all things are equal there will always be an advantage having a higher resolution.

It is like the difference between a �300 CD player and a Meridian CD player. We all hear it, but most people can't as they are expecting night and day results.

Bluray Vs DVD??
Absolutely massive differences, even my Nan can see the benefits and she's blind!!
Seriously think something is not set up properly if the differences can't be seen, just the bitrate would give so much more, even if you output it at 480p to match DVD it will still walk all over it.
It is like the difference between 128kps MP3 and DVD-A.


HDMI Vs RGB.
If the resolution is the same and both inputs are calibrated properly (usually HDMI pushes black too high giving dithering in darker areas and a more washed out look) there should be no difference.
HDMI was a cable designed for the studios benefit, not the end users.

HDMI Vs SDI.
If there is a pure mode on the player which truly bypasses all processing should be the same.
However, only a handful do.

No Darling, I've had it months!