Before I start ranting, I just want to say that GTA4, as a game of its own, is actually quite good, but as a GTA game, it has shortcomings. So Let's kick this review off right.
Graphics: GTA4 has rather well defined graphics. It is definitely much better from it's predecessors. Among many of the improvements graphically; people in the car actually move as they speak, blood stains actually appear on your car if you hit people, hitting newspaper stands actually cause newspapers to go flying everywhere, and some of the text on those flying newspapers is actually legible. So, suffice it to say, GTA is graphically a good game. Is it as good as all the other games out there, not really, but it is an open ended gameplay where you can go anywhere, and as far as that goes, it is pretty great.
Glitches: The game has quite a few glitches, as most of them do, which cause random events from a car to start flying in circles to your legs falling through the floor. It is expected in most games of this style I suppose, but is worth mentioning that these are no fewer than any previous GTA Game.
Setting: Taking place in a new liberty city similar to New York, the game offers little in the ways of variety. You go from tight closed off streets with big buildings to tight closed off streets with really big buildings. It is a bit claustrophobic, giving you very little room to just drive and speed up to top speed. San Andreas was expansive, giving you three cities completely different in appearance, numerous small towns, mountains, forests, and deserts. 4 gives you one city, nothing else.
Car Gameplay: The cars in this game seem much more difficult to control, was this done for realism sake, I am not sure. However, sometimes, some cars have incredibly poor traction and put you into terrible accidents when you think you have plenty of time to stop. It is hard to tell which cars are better, as appearance doesn't always suggest driving style. A couple of added things kind of make the gameplay funner. Most park cars are locked now, so you actually break the window, unlock the car, then actually hotwire it, You can speed up hotwiring, but it usually doesn't take more than a couple seconds anyway. Also, you can receive damage, fly out the window shield, die in fatal accidents, drive drunk (which always gets a cop on your tail) and even have a car die and won't start up again. However, many of these events seem random, and you can ram a building at 50 miles an hour and recieve no damage, or hit a fence and 25 miles an hour, fly our your window shield, and have you car land on you. So that kind of sucks.
Person Gameplay: On a surplus, you can now use cover and hide behind stuff, also peaking out from behind cover to shoot an opponent. You also have a new melee system, which includes kick, punch, and block combos. It isn't much better than the original system, but it does give you a bit more control over what you are doing to your opponent. However, once you get a couple guns, this becomes almost irrelevant anyway. Everything else is just about the same from previous games.
Story: The story is OK. I can't say I feel too passionate about the protagonist, who just doesn't appeal to me as much as the GTA3 + characters, who were frequently hilarious and likable. Niko revenge story does not seem as passionate as GTA3, and that guy was a mute. He lacks the ambition of VC character, and SA did a better job of displaying that protagonists goals and ideals(protecting his home and family). Besides that, it seems less humorous and more serious of a story. It still has all the jokes from the previous GTAs, I can't really put my finger on it, but everything in this one just seems more serious. One reviewer believes that this is GTA with a conscious, which I kind of agree with, maybe groups put pressure on Rockstar to make them show the consequences of your actions, since it seems like you're constantly being reminded that killing, drunk driving, and prostitution are bad.
The Extras: This is where I feel the game fell. Had this game been the direct sequel from GTA III, I probably would have been a lot easier on it, but VC and SA existed, and this game almost seems to have forgotten that fact. This game almost seems like a stripped version of it's predecessors. No gang wars, no gambling, no owning businesses, no car collections. Flying has almost been cut out of this game, although I can't imagine why, it was such a successful feature. Fighting on a flying plane, then para shooting out was the coolest thing ever, not to mention stealing a harrier jet from a battleship. The girlfriend thing from SA was added to, making normal friends too. This is worse, as you now have to maintain relationships with not only your gfs, but any friend you make as well. I also believe the "sex scenes" you get at the end of the date are toned down a bit. I dislike where Rockstar is going with this game, it almost as if they want to take the open part out of open game. They keep you caught up with their characters with frequent cellphone calls, rather than letting you initiate your own sidequests by setting in the right vehicle or shiny circle.
Sum it Up: So in general, this game is a worthy sequel to GTA III... as long as you pretend SA and VC did not exist. It cuts out way too much content added in the last two (what do you call them, expansion packs?). Why they felt the need to cut so much? Or did they feel they had to to make room for more graphics? If so, I would pick side missions and content over graphics any day.
One more beef that bugged me endlessly, they cut out hidden packages and replaced it with flying rats that you can shoot. That was a good idea, but the thing is, you don't get anything unless you shoot all 200, which gets you an achievement (big whoop) and a pop up for just about the only flying thing in the game (helicopter). I always depended on those items that pop up have 10, 30, 50 packages. Just knowing that if I got arrested or used up my ammo, that I always had a store at my place was great. This just bites, what horrible thing happened to Rockstar, whom created a nearly perfect game with San Andreas, feel they needed to take a chainsaw to the game and cut out all of its charm. Oh well, I hope that answers your questions and helps your decision in getting GTA 4.