Episode IV: A New Hope
Lots of matte lines removed. The space under Luke's speeder looks much more realistic.
R2D2's blue parts are much cleaner and more consistent.
The 1997 animation added to the entrance to Mos Eisley is much smoother.
Greedo and Han shoot at the same time, and Han's dodge is far more realistic.
Jabba has been retexturized and looks considerably better. Han still walks on his tail, but it's much smoother.
The lettering on various Death Star panels are written in Aurebesh rather than English.
Diagona has been digitally enhanced with a iris and an eyelid
The matte-painted hall in Leia's detention block looks much more realistic.
Lightsabers look much better. Colors are far brighter and the blades are more defined.
Episode V: The Empire Strikes Back
The battle of Hoth is enhanced with a ground battle between the Rebel soldiers and stormtroopers
Ian McDiarmid has refilmed scenes as the Emperor instead of that near-fossil they had.
The conversations between Vader and Palpatine has a few new lines. Palpatine knows that Luke is Vader's son.
Light sabers are again fixed, making them more consistent.
Temuera Morrison does the voice of Boba Fett
Luke's inane scream when he jumps to escape Vader on Cloud City is gone. And there was much rejoicing.
Episode VI: Return of the Jedi
Like the other two, the light sabers are fixed.
The Rancor looks considerably better, with its matte lines removed.
When Vader's helmet is removed, his eyebrows are gone and his eyes are recolored to match Hayden's still played by Shaw.
Naboo is now part of the celebration
Jedi temple appears in the background of the coresunt celebration
Hayden Christensen replaces Sebastian Shaw as Anakin's ghost. Lucas explains this in the commentary, which is spoilerish for Episode III.