I won't list all the differences between the two films, but I will mention one thing that grabbed me.
Okay, you remember the basic premise of Superman II is that Kal-El gives up his super-powers, so that he can be with Lois Lane.
And shortly thereafter, Clark Kent and Lois Lane go to a truck stop where a mean sunuvabitch trucker whups Clark's butt.
In Richard Donner's cut, we get all that, but the ending is very different. Instead of kissing Lois and erasing her memory of the past week's events, Superman flies into the sky and circles the Earth, making it turn in the opposite direction, causing time to move backwards. Yes, the same trick he used in the first film -- only here, instead of going one hour back in time, he goes several days back in time, essentially un-doing the events of the movie. The whole thing is very Donnie Darko. (And by now I would've expected someone to upload the sequence to YouTube complete with the "Mad World" song) It would've been funny if instead of Superman un-doing all the events, it was Richard Lester (ha ha.)
So by the end of Richard Donner's cut, the three Kryptonian super-criminals never arrived on Earth, Lois never learns that Clark Kent is Superman, and (presumably) the kid with the negligent parents plummets to his death in Niagra Falls.
But here's the kicker: Clark goes back to the truck stop and beats the hell out of the truck driver.
Even though, by the film's logic, the truck driver never messed with Clark Kent. Superman hurls that trucker down the countertop and smashes his head into a pinball machine.
The truckers and wait-staff must have wondered what the hell promoted this Bernard Goetz-looking mo-fo to come in and assault a patron for NO GOOD REASON.
As the website says, Superman is a jerk.