Take two DVD players hook them both to your HD set. Take one and use the HDMI (720p/1080i/1080p) port and take the other and hook it up using the composite (yellow) video cable. Now watch a snippet of any DVD using both (all DVD's are mastered at 480i). Tell me that there isn't a world of difference... not only in detail and sharpness of image but in depth of color.
Well of course it's going to look like crap if you use the composite video cable. But who in the hell would do that if they have an HDTV? Any decent DVD player is going to be connected using either HDMI or component video. There's really no excuse not to, if you have an HDTV.
But that's not what this is about. All DVD players worth their purchase price will at least have component video connections (if not HDMI), but at the same time the HDTV itself is likely to have a better upconverter. Some, if not all, of the 'upconverting' DVD players will only do so over HDMI, anyway.