I spent a lot of quality time with GTA in the last two days, and it's really starting to set itself apart from previous entries in the series, in some really good and interesting ways.
The importance of interpersonal relationships has been increased a lot. You're encouraged to spend time with your business partners, and doing so generally reaps some benefits. Once you're in someone's favor enough, you'll be able to take advantage of their unique special ability. You also can date characters in the game. From time to time, your contacts or love interests will give you a call, or you can give them one, and you go for a date. You can take them bowling, to play billiards or darts, to a cabaret or comedy show, or you can just go out drinking. But, (and I echo this statement for real life) be sure to call a cab if you do the latter though, it's very difficult to drive drunk, but it's a fun minigame to try once or twice.
But even if you're not hanging out with someone, there's a lot to entertain you on your own. There's a TV in your apartment with a number of different channels full of parody versions of your favorite shows. There's an internet cafe that you can browse GTA's version of the internet, again with parody websites, blogs, and spam. And again, the radio is a big star, with great talk radio, and there's a feature with the new Rockstar Social Club that you can flag tracks in the game for purchase later through amazon.
Perhaps the thing that's going to keep me coming back more than anything else is the story. So far it is pretty enthralling. Niko (the eastern European main character) isn't just some mafia thug, he has a dark past and an interesting backstory, and a conscience. Some of the missions provide you with a choice, and you can decide that what you're doing just isn't right, and the story will take branching paths based on the choices you make.
There's also a fully featured multiplayer component to the game, so if you've ever wanted to run around Liberty City wreaking havoc with friends, you finally can. I haven't been able to investigate that yet, I'm waiting for my friends to buy a copy and join me, but their going to wait 'til after finals. Jerks.