Opticians don't test vision in SD or HD. There is also the theory (which I think is right) that women can't tell the difference. I know I can't.
My wife couldn't at first, but she now grudgingly admits she can see a 'very very slight difference'. Not enough to make her switch channels though, she'll quite happily watch BBC1 in SD, whereas for me it looks almost like old-fashioned analogue CRT 405 lines. Maybe it is more of a bloke thing.
O/T - who remembers the effects of the CRT raster rate when TVs shown on TV (like a news report from a newspaper office or the stock exchange) would have a big black line slowly going up them? Or when things like wheels on wagons in Westerns appeared to go backwards?