Here are the changes that I know of some are not deliberate:
ANH - Luke's saber is green when he practices, this was not deliberate, it seems that the color and film enhancing techniques the quality of DVD is showing this, this way, for the first time. I think originally it was more white, and of course when Obi Wan gives him the saber for the first time and on Empire Strikes back it's blue.
ANH - Han Solo and Jaba the Hutt scene have been improved dramatically. Jaba looks better, and Han's shadows are more enhanced.
ANH - Obi Wan vs Vader - green flashes of light instead of white, same thing with the sabers listed above.
ANH - Many people spotted the goof where the stormtrooper bangs his head on the low door, Lucas threw in a sound effect for the mistake now. Nice nod and poking fun at himself.
ANH - Matte lines on a properly calibrated TV are totally gone IMO, and in 1997 I thought the difference between the new CGI elements and the old models was like night and day, for some reason it seemed to me those lines were blurred. It also seemed to me like the Death Star was CGI, because the detail as the Millenium Falcon was approaching it was outstanding, no confirmation on this though.
ESB - James Earl Jones redid some of his lines, including "I am your father" (damn I should have spoilered that in case you haven't seen the movies... yeah right).
ESB - More Matte line improvements
ESB - The scene between Vader and the Emperor completely redone. The "new" emperor from the prequels did the scene. They also redid the dialogue.
ROTJ - Mattle line improvements
ROTJ - Final scene they insert Naboo in as one of the planets celebrating the fall of the Empire.
ROTJ - Hayden Christiansen fades in as "Anakin" at the end. The reasoning according to the DVD commentary (I haven't heard it just read another's post) is that, the last time Anakin was "Good" was his 22 year old self, so remains that way as part of the light side of the "Force".
That's all I got so far. Remember, these are additional changes FROM the 1997 Special Edition release.