Then the other massive change was "no one can be offside when the ball is played" to "no one in an offside position can attempt to play the ball". Totally changed the way the offside trap was practiced, I believe.
The biggest change in my memory was the one that stopped the endless passes back to the keeper by preventing him from picking it up if it had been kicked back. Also worth mentioning is the 3 points for a win I suppose.
EDIT: This is what I was thinking of:
https://en.wikipedia.org/wiki/Offside_(association_football)...
Doesn't say when the change in interpretation was made.