A great rule change was the one that stopped you wrestling a guy who had just taken a mark. Prior to this rule, someone would mark a ball, and their opponent would hold them up long enough for the defence to get back.
And I don't know what rule change stopped players coming from behind and punching the head of the guy going for the mark. Watch games from the 1990's and you'll see all the players who led and marked duck their heads the instant they had the mark - so as to avoid a fist in the head from behind.
If I was allowed one rule change for next year, it would be the "deliberate out of bounds" rule. It would become a free kick for any kick that went out of bounds without being touched, and a free handball for any handball that went out of bounds without being touched. The rule would only apply where, had a successful tackle been laid on the person to kick/handball out of bounds, it would have been holding the ball. Everything else is a throw in. Essentially it stops the idea of "disguising" an attempt to get the ball out of bounds.