On a properly configured TV, it is easy to tell the difference. My grandparents (85 and 90 years old) can tell the difference between HD and non-HD. They were actually surprised and excited by what a big difference it makes. My grandmother especially got that initial HDTV euphoria, where anything, even sports, is fun to watch as long as it is in HDTV.
The problem is that most folks can't properly plug in and configure an HDTV.
The problem is that most folks can't properly plug in and configure an HDTV.