GTA3 is good because it offers the opportunity to have emergent gameplay. ie: It is a virtual arena with set rules, populated by permanent AI characters who have set patterns and reactions. Combined with a large environment and numerous 100+ missions it offers very open ended gameplay. This also lets a gamer decide how they want to solve puzzles. This is very important for a immersion in a game. Most importantly it does it all with the illusion of non-linear gameplay. While it seems you are free to do whatever (and you are), to progress the story further one must complete certain missions. GTA3 is a very well constructed game that many companies are going to emulate in the future. NOt the actual style or genre but the manner in which the game is constructed and designed.