Missions are more fun and have better structure
While V had more variety with missions, many of them were filler. You wonder, why am I doing this?
The heist prep missions come to mind. In one heist mission, you have the option to either fly in, hack a computer and have a shootout. Or disguise yourself as a janitor and mop. Mop. Realistic, sure. In execution, boring.
One of the worst missions in GTA V is Scouting the Port. Basically, you're undercover as a freight worker. And you have to use all this heavy machinery to progress. It's slow, mundane and tedious.
One underreported aspect that the 3D era games had was freedom. This video highlights what I mean.
Basically, the simplicity of the 3D era allowed you to complete missions more than one way. Let's use III as an example.
One mission had you kill a target in Chinatown. Okay. You can run in, spook him and have a car chase with him. OR you could shoot him from afar, or take his car, prime a bomb to it, park it back and lure him to the car.
Another aspect that the 3D era games done well: exploration. There are guns, collectibles, health, armor, side activities hidden inside the city. They're put within the nooks and crannies of the world to reward players for going off the beaten path. Open world.
The game gives you tools and it's up to the player to figure out how they work. Is it perfect? No. Have they aged well? No. Are some missions a drag? Yes. But don't knock an older game just because it's "older."