Lack of oxygen and freezing? Don't forget the total loss of atmospheric pressure, he would have exploded, and we certainly wouldn't have been able to hear his voice because in space there is no air to vibrate to make sound waves! But the parents of the kiddies don't want that. :P
And yes, haha, FOR FREEEEEEEE???? is possibly the best moment in the whole film, I always rewind it to play it again!
The mirror... it's broken. Yes, I know. I like it that way. Makes me look the way I feel.
The best moment by far is when his niece comes in to say "merry christmas uncle" to him and he spins round in his chair, the camera brilliantly going close up at the time and he says "IT CERTAINLY SHOULD BE, AHAAHAHAHAHAHA" with a devilish sort of grin on his face, absoloutely hilarious.
It still kind of freaks me out. But that's the Salkinds for you; they like to play rough. See also: Supergirl where the relatively harmless Brenda Vaccaro is forced into the Phantom Zone with Faye Dunaway at the end.
For that matter, the original scripted ending of Superman II had Superman hurling Lex Luther and the Kryptonian villains into outer space where they, too, would presumably die from exposure.
The Salkinds made this movie they made the supermans as well like Superman 4 when Nuclear man carries off that woman into deep space and she seems just dandy.