here's my proposal for a definition of special-edition-worthy changes: Changes that are so obvious that most people notice them and that affect the storyline and the character development. Excluding new scenes, these are not changes to existing scenes but only additions.
(I know, it sounds confused and contradictory so if someone can find a better one, please go ahead)
Indiana Jones: not SE
TPM and AOTC: not SE
Blade Runner Director's Cut: SE
SW SE: duh
another question: why didn't George Lucas call the SE "Director's Cut"?