We don't have an official list yet (Adywan is working on it, I think).
Basically, my understanding is that it comes down to an un-Special Edition with some tweaks here and there.
The 2004 Emperor voice will remain, but the dialog will be modified to match as closely as possible the original dialog. The bad Revenge of the Sith makeup job will be replaced with modified video/images from Return of the Jedi (see the ESB Revisited Teaser Trailer).
Windows in Cloud City will remain open for the most part, with windows that disappear all of a sudden remaining open.
Boba's voice will be changed back to the original, and Jango's helmeted voice will be modified in some way in Attack of the Clones Revisited.
There will be no noticable score, plot, or pacing changes as for the most part ESB is already perfect.
Now this is all to my understanding of Ady's past posts. There might be other changes he's willing to share shortly. Part of me wants to know every tweak he's going to do, the rest of me wants to be surprised. I think the "know-everything" part will win though. It's in my nature.