It takes the structure of the first games and greatly improves upon it.
Combat is greatly improved with the application of a system of hit points, and more accessible body armor. The weapons are more varied (at least in level 2 and beyond), each with their own strengths and weaknesses, and in addition to this, car mods and better vehicle controls improve the "twisted metal" aspect of the game greatly.
The system of gangs, though far from perfect, is a great addition if you really thing about it as well. Why? It is another group of characters thrown into the game.
You have the mindless masses who walk about aimlessly (on side walks of course), the occasion red wearing muggers and green hijackers (ever get hijacked during a mission? that is a great addition if you ask me), cops (who, like the mindless masses, travel about, mindlessly unless something arouses suspicion), and now gang members (who, if you're in bad standing with, will attack you for just being there). Long sentence aside, this adds a new dynamic: you get shot at for not doing anything necessarily bad within recent memory.
In addition to this, the city is active. Cops/gang members will peruse (and murder, lol) muggers and carjackers, carjackers can be kicked out of their stolen vehicle by a pistol wielding driver (it's nice that not all citizens don't let themselves be walked over). It's simple, but it makes the world feel more active.
It's these additions, and the ability to SAVE YOUR GAME, that make this game an entertaining experience.