Here’s some of the changes I like…

The expanded entrance into Mos Eisley. It really changes the tone of the original film’s first act, because Obi-Wan & Luke hiding out in a bigger place would make it harder for the Stormtroopers to find them.
The improved X-Wings during the Battle of Yavin. Most of the older models still hold up extremely well, but its still awesome that they included some new angles along with added lasers and such. It makes the whole sequence feel less repetitive from a visual standpoint.
Both enhanced explosions for the Death Stars. Some have nicknamed them as the “Praxis effect”, referring to the opening of “Star Trek VI”. But I think what was done in the Special Editions was even more impressive.
No more matte lines or transparent cockpits during the Battle of Hoth. The ships look much more realistic that way.
The expanded approach to Cloud City, and the windows added to the interiors. The original version just looked too obvious that it was filmed on a confined set.
Ian McDiarmid being added in as the Emperor’s hologram, though I personally wish they’d have made him look as he appeared in “Return of the Jedi”.
The alien “beak” and tentacles added to the Sarlaac Pit sequence. The original version just wasn’t too threatening; it was an open mouth with teeth and little else. Having the creature attack our heroes added some necessary tension.
John Williams’ new music for the ending on Endor, with all the other planets shown celebrating as well. The original “Yub Nub” song was okay as a kid, but as I grew up it became annoying to me.
Hayden Christensen being added as Anakin’s ghost. I know a lot of people will disagree, but since the prequels were still being made at the time and George wanted different things to rhyme, it made some sense for the narrative. Its not like she completely reshot Sebastian Shaw’s “dying Vader” scene; that would’ve been completely disrespectful by comparison.