I was never particularly good at volleyball, except within the context of the stake men's league we used to have at church. However, I have always liked to watch good volleyball being played, so it interests me whenever rule changes are implemented. Four significant rule changes that have occurred since I played are (1) the "legalization" of the two-hand "soft dig" (or "beach dig," as we used to call it), which was always called a double-contact in the old days, (2) a serve's being called "in" that hits the top of the net and falls over into the opponent's side of the court; (3) the ability to serve from anywhere on the back line; and (4) the creation of the "libero" position (a back-row player, a sort of "designated" digger or serve-receiver, who can be inserted at multiple positions in his team's rotation). Two additional changes in strategy, which did not entail major rule changes but have changed the game greatly, are (1) the jump serve (which in turn has placed a premium on passing skills); and (2) widespread use of the "5-1" system, where one setter sets from all six positions in the rotation (which in turn requires one attacker from the back row when the setter is playing in the front row).
However, the rule change that, in my view, has had the biggest impact -- an extremely negative impact -- is the implementation of so-called "rally" scoring, where every serve produces a point. (In the old days, only the serving team could win a point -- the other team played to win the serve.) I guess the rationale was to speed up the game, which it definitely has done, despite the fact that it now it takes 25 or 30 points to win a game instead of 15. However, it also makes it extremely difficult for a team to come back from any significant deficit, because the team that's ahead only has to keep "siding out" to win the game. Also, given that it's generally much easier to win a point when receiving serve than when serving, "rally" scoring tends to punish a team that has won a point by making it serve the next point. The logic seems backward to me: if the rule-makers really want to speed up the game, why not make a team serve until it wins a point, then make the other team serve until it wins a point, and so forth? It is for these reasons that "rally" scoring has practically ruined volleyball in my opinion.