The only things I think that weren't possible due to technical limitations back then were:
*Luke's Landspeeder without the 'forcefield' and blurry vehicle shadow.
*The Jabba the Hutt scene in ANH (although I think even that was only left out because they hadn't decided what Jabba should look like, fact is that the technological hurdle arose when they decided to make Jabba a huge slug-like creature in RotJ). The scene in ANH was left out on purpose back then, not because of technical hurdles, imho.
*More agile Dewbacks.
*The Jawa riding on the huge beast of burden.
*Fancier blastoff of the Falcon from Mos Eisley
*Censoring the violent scenes aboard the Death Star shootouts
*Adding the "Close the blastdoors!" line to the stormtroopers (Hehe, in fact that seems to be a goofup from the sound-editors,
because that line is in the 1977 version of the german voiceover. So it was truly intended to be in there from the start.)
*Rebel ships starting from Yavin IV.
*More vivid space battle during the assault on the Death Star.
*Extended Whampa creature footage.
*Toned down opacity of snowspeeder cockpits.
*More flexible camera paths during the Falcons approach to Cloud City
*Mass scene of people listening to Lando's announcement.
*"Jedi Rock" Musical scene in Jabba's palace.
Everything else could have been done in the first place without much of a hassle, imho. Lucas is just not being honest about this and I really can't understand why. It almost feels like he knew he'd be sneaking in stuff that fans wouldn't like. Instead of being honest and saying "Well, I changed my mind about some of the decisions and compromises I made back then and they don't fit in my Star Wars agenda nowadays, so I went in and adjusted them.", he hides behind the 'technological limitations at that time' statement as some sort of universal excuse for every change he made.
Many of the cool things that got into the special editions (meeting Biggs Darklighter before the Death Star assault, extended Whampa scene, removing the 'forcefield' from Luke's landspeeder and toning down the opacity of the snowspeeder cockpits) is just the sugar added to the mix to distract from the scenes Lucas changed his mind about, imho.
Also, even if he would be worried about the OT outselling the SE, he could avoid that very easily by including the OT with the SE in a big double package. That way the OT sales could simply be added up to the SE sales.
Edit: Refrased post fundamentally.