I think those people are largely in a minority, whether it is on a film forum or just in the general public. If you can't see the difference between a DVD and a Blu-ray you need your eyes checked. Even on a 21" monitor the difference is night and day unless you are comparing a poor example of a BD, in which case I would agree the difference is more subtle.

I think the best educational tool is actually broadcast HDTV, like sports and the like. You can switch from one channel showing it in SD to another channel showing it in HD and it's like two different shows. When my family members did that, just by chance, the true quality of HD hit them (and HDTV broadcasts usually aren't good compared to BDs).