i generaly find that the resolution is much better (Moar pixles) the frame rate seems smoother and in some cases the framerate is higher.
but if the source was only 480p and then riped as a 1080p you wouldnt notice the difference except for the bandwith cost.
but if the origional copy was of superior quality and had to be compressed to 480p to fit on a conventional dvd then the 1080p is better as it would contain moar information than the 480p (sounds like im going in circles)
but if you take a real movie like transformers.
the standard dvd looks good.
but watch it on blue ray and you would be like

same goes for a image you can have a poor scan of an image and try to upscale it. and it looks sht.
or you can have a 1080p scan of the orginal and it would look so much better.
i hope i havent strayed XD