It's the gameplay, stupid.
I still remember beating a particularly difficult mission in GTA: Vice City back in college. You have to go up a staircase to a landing on a building where you can get a vantage point to take a picture of a politician canoodling with some bimbos. You are spotted, and need to get back to your safe house. Unfortunately, that politician has called in some favors, and now the police and FBI are after you. Just little old you as heavily armed cops storm up the staircase and pursue you with helicopters and squad cars. How to do it?
In a lesser video game, there would be a "right" answer. The makers of GTA, however, don't really care how you do it. Get yourself a heavily armored transport and go up the steps with a camera and an assault rifle? Go for it. Try to make the run across town in a Ferrari? Take your best shot.
I did it with a motorcycle and a helicopter. I drove the bike up the staircase, took the picture, and then drove back down, taking out police by running them over and firing an uzi as I went. At the outlet from the building I took so much fire that my bike was immediately set on fire. I continued driving the burning bike until I was close to my helicopter, and then leapt off. The bike continued for a few more feet and then exploded. I got in the copper and flew it across town until it, too, was on fire, before I got to my destination. I jumped out of the chopper outside my safe house from 50 feet up, nearly killing myself. The chopper exploded midair and crashed close by, and I ran the rest of the way without getting hit by the last bullet that would have killed me.
The violence is the hook that catches you, but what keeps you playing well after the novelty of being a thug wears off is the infinite possibilities and incredible detail. Playing the other day, I got into a head-on collision, and the occupants of the other vehicle shot out through their windshield. I blocked a tunnel with a dump truck and took out a police escort with a rocket launcher. I went on two dates.
Too bad this was all at Jim's house. I need to get a job so I can buy a PS3. Hmm. Maybe I can steal a few cars...