The major changes on the Star Wars Trilogy DVD's

1. Luke doesn't scream anymore, when he jumps off the platform at Cloud City in "Empire".
2.  Boba Fett has a new voice. Since Boba Fett is a clone of Jango Fett, and Jango Fett is played by Temura Morrison, it does make sense that he should sound like him too.
3. A new and improved Jabba in "Hope". The CGI model has been improved and looks much better now.
4. At the hologramm scene in "Empire" , Ian McDiarmid (who portrays the Emperor in "Return of the Jedi" and the prequel trilogy) now appears in that scene with a little more dialogue. Originally a wife with mask played that part and the voice was from Clive Revill.
5. The light sabres in "Hope" have been improved (blade color is more pronounced and the animation is more like the prequel films).
6. The Han/Greedo fight in "Hope" does look more natural, Han's head doesn't jerk like it did before. Instead his whole upper body shifts over slightly. And the blaster shots are very close together now, so the encounter is not as implausible as it was in 1997.
7. When Luke removes Vader's mask in "Return", to reveal his father's face, the eyebrows of actor Sebastian Shaw have been erased (presumably because Anakin's face will be badly burned in Episode III).
8. After the second Death Star is destroyed, when the film cuts to the various celebrations around the galaxy, we also get to see the planet "Naboo".
9. The most noticeable change (and perhaps some "Star Wars" diehards fans will get really angry about it) is Hayden Christensen (who plays young Anakin in Episode 2+3) now appears in the scene, where the "force ghosts" of Anakin, Obi Wan, and Yoda smile at Luke at the end of "Return".
   
   

There are also some minor changes with the sound mixing too, but the main changes are definitelly the ones viewable.

Source: CNN / The Digital Bits