All the posts I view seem to imply that the observed lower quality on an HDTV is due to the stretching of the picture on a larger screen. This is not the case. I have a 39" CRT SDTV and a 47" HDTV and the size of the picture of a standard definition display is actually slightly smaller on the HDTV. But the same signal that looks clear and sharp (sharp lines and fine details), looks cartoonish on the HDTV.
The problem is the the converter is doing an extremely poor job of determining how to display this on the different number of pixels available. I have found this to be the case on every 1080p TV. Standard definition just looks awful on them. Everyone has the misconception that it is just the size difference, or now that they are spoiled to HD the old display looks worse, but it always was like that. It is very easy to see that is just not true if you have both side by side. Is it just that a decent upconverter just wasn't considered important? If you expand a 640x480 graphic to fill out the screen using a computer input, you will see none of this blurring. The graphics engine in a computer does what an HDTV processor does not do.