The devil is in the details... and GTA IV is possessed.
GTA IV takes all of that, cranks it up a couple of notches, adds some grit and realism, and introduces vastly improved shooting mechanics, a driving engine that lends real weight and inertia to vehicles (which hearkens back to the brilliant Driver on PS1), a surprisingly fleshed out online multiplayer experience, and some of the best physics I've ever experienced. Fans of GTA are really getting their money's worth with this one.
I'm not going to go into the story line here (I don't see the need for such uber-hyped titles), since you probably already know enough already. I will say you play as Niko Bellic – a man of questionable moral character, hailing from Eastern Europe (most likely Serbia), who comes to Liberty City, America to make a better life for himself… and ends up getting some sweet revenge along the way. The main story will probably take you over 50 hours to complete, assuming you don't blaze through at top speed. And how can you? With such a fantastic city at your fingertips, you'll find yourself getting lost in the scenery for hours at a time. Plus, with so many ancillary characters, side missions, entertainment hot spots (such as the brilliant comedy club - Ricky Gervais FTW!!1!), etc. to enjoy, you're looking at hundreds of hours of possible content. I beat the game in 70+, and I've barely dipped into the online portion.
Speaking of the online portion, forget about "maps" or "levels." There's you, your opponents and teammates, and Liberty City. That's right, all the burroughs, all the vehicles, all the pedestrians, EVERTHING. If that just doesn't scream "Awesome!" then you must have wheat grass running through your veins. There's a ranking system, character customization, and many types of game modes, ranging from your standard deathmatches to team v team objective-based rounds. There's also a free roam option, where you and your buddies can just run around doing whatever the hell you want. It's all very fun stuff.
The one thing that has not been kind to GTA IV is the historic flood of perfect review scores. Yes, these scores were undeserved. It's hard to know the real reasons behind such a phenomenon, whether financial incentives or "fanboy" reviewers were involved, I honestly don't know. What I do know is, as awesome as many of the missions are, they do get repetitive. As awesome (and detailed... and huge) as Liberty City is, it's not the prettiest environment you've ever seen. Your NPC friends call you A LOT in story mode (that's right, you have a cell phone), to the point that you'll find yourself just shutting it off to explore the city in peace. The story is great and engaging the whole way through, but some of the characters you encounter are too annoying or over-the-top and work to detract from the otherwise high-quality of the plotline. I also found the radio stations to be of less interest than in other entries (Vice City being the crème de la crème).
I could get into a lot of other stuff, but one can quickly glean such information from a million other sources. My reviews are my opinions, and my opinion is that this is the best GTA yet and a great game that everyone should try. It starts out slow; so to all you newcomers, give it a few hours if you find yourself initially disappointed. Does GTA IV live up to the hype generated by all the perfect 10s? No, but it stands as the best the genre has to offer nonetheless.