I was just listening to the mono mix of SW again, and noticed differences which nobody seems to have mentioned before.

In general, due to the narrower frequency range of optical Academy mono (100Hz-8kHz vs. Dolby Stereo's maximum of 30Hz-16kHz), certain sound effects are vague or nonexistent; the low-frequency rumbles and high-frequency whines are dulled considerably. (For example, compare the Tantive/Star Destroyer chase in stereo and mono. The high and low frequencies just aren't there in the mono.)

Here is an ongoing list of differences I've found, with GOUT timecodes. I'm using the original Dolby Stereo mix as a basis of comparison, not the '93 mix. (A warning: The GOUT-synched Belbucus reconstruction is not a 100% accurate reference, so I will note if anything can only be heard in genuine mono sources like Puggo Grande.)

00:00:00-00:00:18 - Fox fanfare has less reverb (Puggo Grande has the actual mono mix fanfare; Belbucus reconstruction uses a mono fold-down from the soundtrack CD)
00:01:59-00:02:16 - Various differences in flyover scene; Tantive's lasers are louder, flyby roar/whine is lower; Star Destroyer lasers are quieter; music seems to be more to the front of the mix, especially when Star Destroyer is flying overhead
00:02:16-00:02:22 - Star Destroyer lasers are louder, Tantive flyby roar/whine is lower (possibly also due to lack of high frequencies)
00:02:22-00:02:25 - No sound effect for laser blast that hits the Tantive; different explosion sound
In the interrogation scene, Captain Antilles' dialogue is edited differently. In the stereo mix, it sounds like "We intercepted no transmissions! *ugh* *ugh* This is a consular ship...(pause)...we're on a diplomatic mission!" In the mono mix, it's "We intercepted no transmissions! (pause) *ugh* *ugh* This is a consular ship...we're on a diplomatic mission!"
When R2 falls over after being knocked out by the Jawas, he makes a different sound effect when he hits the ground. (In addition, there's an extra sound effect at the end of the previous shot after the electricity animation ends; it's a metallic "bang".)
The background Jawa dialogue is a lot louder than in the stereo mix. (At first, I thought the mono had extra Jawa chatter, but it is in the stereo too, it's just much quieter.)
00:20:47-00:22:37 - When the hologram appears, an added sound effect can be heard of a metal object falling to the ground. The hologram itself has a new sound effect as well as added sound distortions when the image breaks apart. When Luke removes the restraining bolt a new sound effect is added when it shuts down the hologram.
00:34:44-00:35:21 - Again the hologram of Leia has a sort of projection sound (the static distortions were already in the stereo mix)
00:51:25-00:51:31 - Creature vocalization added, heard at the moment when Luke has sold his speeder, more exactly when you see the legs of an alien passing by in front of the camera.
01:25:02-01:25:04 - Luke's line "What good will it do us if he gets himself killed?" is a different ADR take.
01:42:52-01:42:57 - "Lock S-foils in attack position." Added X-wing sound effects.
01:46:08-01:46:10 - "There's a heavy fire zone on this side" can be heard more clearly due to the lack of distortion on the radio transmissions.
01:50:17 - 01:50:24 - "Stay there. I just lost my starboard engine." Added sound effect for Red Leader's damaged ship.

I think we should try to find all the differences in the mono mix, sort of list of differences in the 70mm version of ESB. I'm sure there are lots and lots of other subtle mixing and editing changes, some of which may be really hard to spot, but I think we can try to find as many as possible.

I don't have the time or patience to do this all myself, so if anybody else can provide any more differences they spot, preferrably with timecodes (for -1), I'll add them to this top post.