Is it a result of upscaling for HD display
That would be my primary suspicion, but I don't have empirical evidence.
My own experiments show that DVDs should be watched on standard definition CRT sets, and only Blu-rays (from HD source) should be used with HDTVs. When you violate that rule, you can get a less than ideal viewing experience.
I've also seen visual problems, when DVD-Video material is upsampled to an HDTV. Some players do a much better job of this, than do others. Would assume that it is the upsampling algorithms used by the various players. My relatively inexpensive Panasonic seems to do a better job on a 37" Toshiba, than my much more expensive Marantz on a 55" Zenith. Could also be both the size and the differences in the display devices too. Have not done any A-B comparisons by removing the variable of the display, so the preceeding is in no way scientific.