The sum of its parts...
What needs to be said probably already has, and better, but lets just say that GTA is one of the most singularly enjoyable games of all time. Even when you really delve into each aspect of the game, none of the gameplay elements really stands out as exceptional, and if you took just one gameplay element from the game and tried make a standalone game out of it, odds are that game would be getting a score somewhere around a 1 out of 5 (or lower, if even possible).
Driving and stealing cars is probably it's best aspect, but everything else in between is almost scary bad. Shooting is haphazard, AI is about as smart as a newborn piglet, and the story is a disjointed mess. And yet, take all of these half baked elements and wrap them in a metropolitan environment that really has no equal, and you come out the other end with a very magical experience. It's a toy world just tempting you to go wild within it.
That's GTA3's greatest and most groundbreaking accomplishment, making it's world breathe with a real, almost tactile, sense of being. One of the real joys of the game is just driving around, taking in the sights and sounds, and coming to the realization of just how much time and care was put into the design and layout of Liberty City. Everything that should be in the environment is, and better than that, it all makes sense. Liberty City (and honestly every GTA since) IS GTA's greatest and most interesting character. For that it gets HUGE props.
Along with it's living, breathing, metropolis also comes it's open ended gameplay. The standard by which all since have been measured. The amount of time that can be spent ignoring the missions/storyline and just cruising Liberty City creatively thinking, wondering, "Can I get up there?" or "If I get up there, and then I decide to chuck grenades on that group of pimps down on the street, will the police be able to get me?" is one of the biggest appeals of this game. Exploration and the overall sense that the city is just there for you to "mess with" is probably GTA's greatest gameplay mechanic. I often wished that the consequences for my actions held greater weight over the long haul, but the overall sense of causing mass hysteria and then trying to "get away with it" is one of the best stress relievers since Asian massage.
The storyline is really nothing to write home about, but is a necessary evil if you want to unlock the entire environment of Liberty City and a few extras. Most missions are minimally difficult and consist of pretty standard fetch quests and escort missions. Things start getting more interesting and more varied towards the end, but that's not really what you'll be playing the game for. Sight seeing and exploration is the thing that sells this package, not it's canned mobster wannabe storyline (which could probably be completed in just a hand full of hours to someone dedicated enough. It took me 5 years though if you need a scale for balancing :) ).
Overall GTA3 is a game that immerses you in it's environment so deeply that you learn to ignore it's sizable flaws. GTA3 is the video game equivalent of The Beatles. Everyone knows who they are, some people feel they are both over hyped, and yet a lot of things wouldn't exist today without them. Both are pioneers. Love them or hate them, but you have to respect them.