biggest change was that every other GTA game had a generic crap story with generic crap missions
i've played every single GTA since PSX and i never ever enjoyed or cared about the missions or story. they were always horrible marred by bad gunplay. the only reason to ever do missions in any other GTA game was to unlock more map, cars, houses, etc.
GTA V, on the other hand, has probably one of the best crime stories or crime missions/gunplay in any video game IMO. the gunplay, at least on free aim, is some of the best, the mission variety is far beyond any other crime or open world game, and the story is a real story, not just a hogwash of funny cliches randomly tied together like VC, SA, IV. SA at least had a decent story while in the first city but they it became a bunch of random joke characters serving as excuses to move you to other cities while doing the same generic missions with the same generic bad gunplay.
i absolutely love every other GTA game and especially VC and SA... but i love them for their free roaming, their atmosphere and music, their open world gameplay. the stories were just joke stereotypes created to give you a reason to repeat the same generic missions you've done all game. no real drama or movie would ever use a plot like that.
every other GTA first designed its world, then its gameplay, and THEN they must have decided on the missions, and then finally, decided on funny stereotypes and jokes to serve as excuses to do said missions in said locations
GTA V on the other hand feels like they crafted it like a movie or drama. they started with the story and the plot, then the world, and then decided on the story events, and then chose missions to fit within that story, and finally created exponentially more mission variety than all other GTA games combined to go into those missions.
the story may be cliche... and have its share of stupid writing, too... but it feels like a real story at least. not just something they invented after they finished the game to give you an excuse to do stuff in the game world.