I’ve just come across this video that demonstrates the clarity and definition of HD. The BBC filmed this for a show called South Pacific, but what’s cool is they decided to upload multiple versions so you can compare low to standard resolution and then on to 720 and 1080p high definition.
Best thing to do is click on full screen mode and then choose the resolution. I purposely tried a low one and then cranked it up to 1080p and it is quite breathtaking.
UPDATE : it seems like the embedded player here does not support multiple resolutions, so try going directly to the video on YouTube.