GTA IV is a great game, everybody knows this. Being a great game I feel the need to stake my personal claim in it and just use it as an excuse to repeat things that I've already said. That being: video games allow for a different sort of expression than other art media can allow for.
I'm tremendously impressed with the scope of this game. When I heard people say such things I assumed they were talking about the number of streets, realism of textures and environment layouts, the complexity of the foot and car traffic, the usual GTA stuff. What I've come to realize is that beyond that there's a huge amount of smaller detail put into it. An enormous amount of audio and visual commentary on media and urban culture. It's this smaller detail that actually is what excites me about the game.
Prior GTA games, and sandbox games in general, have had to be large in (virtual)physical scope to allow for experimentation and invention. The only thing they've managed to really reinvent, however, is basic gameplay. GTA III is exciting due to the fact that you are able to just run around and not actually play the game, but all that running around is still set in established platforming and racing paradigms. It's not really special except for the fact that you can just pick up and play in a non-linear fashion.
GTA IV has actually broken into the realm of social commentary in a way that only video games can do. It's paving the way for games to express certain things in ways other media can't.
GTA IV is actually approaching a virtual reality world. The game is detailed to the point that just existing in it is commentary on city life and social norms. None of the easter eggs are too hidden or too expository. They're not so complicated that you'd be distracted by them, but are detailed enough that you can spend an entire play session just on the internet or listening to the radio.
I'm very excited about this game because it's better than a satirical blog, better than a gag youtube commercial, better than game filled with pop culture asides, better than a comic book caricature of a city. The ability to have an agenda and move freely through the environment makes the commentary more real and more poignant than it would if the situation was more static.
Bottom line here is interaction in and of itself is a valuable expressive tool. A lot of this game is clearly designed as commentary, to be noticed as satire, not simply as entertaining. By being able to interact with this environment at your leisure, and by having so many different things so detailed, you are able to selectively and efficiently experience the aspects of this world that are meaningful to you. Whether you take in the gestalt feeling of the city as you play missions, wander around looking for graffiti, drive around listening to the radio or go looking for the building architecture, so much time has been spent on so many aspects that each one has appreciable depth and insight. And since the world is still one entire piece, it's all fluid and cohesive. You are guaranteed to run into things you may not have been interested in and be exposed to criticisms you may find shocking or provocative. It's like an art gallery where you come for a particular artist but end up noticing others passively.
I don't think this would be nearly as effective if it wasn't a game, either. As much as a lot of this game's value comes from the non-goal oriented extras, it has sooo much more charm than Second Life, which is arguably far more radical, but not necessarily more influential. The fact that the game has a decent story with a likable character is just another aspect of the game's various artistic and expressive avenues, but it's also a key component in motivating you through the world. There could be the same amazing world with the same amazing detail, but without a story and goals, who would play? I don't even mean this to sound like "you have to throw people an entertainment bone to get them to sit down and think about something." The story is like a path of arrows guiding you through a dauntingly honeycombed museum. It's too easy to just wander to the most familiar and attractive aspects of a completely open environment, and too easy to zone out of a continuous narrative. The game aspect of this work is a pivotal aspect of it's strength.
You go Grand Theft Auto IV! I spell your whole title out with pride!