I too believe GTA 4 doesn't deserve the scores it gets.
First up, the euphoria (i think is the name) engine, while increasing the level of immersion the player has in the game (nearly everything you run into will have a reactionary effect) it certainly is far from perfect or even polished. There are some instances where it gets ridiculously unrealistic (i.e my sports car jack-knifing on a turn, seriously, I don't think anything aside from truck sized cars tip over on sharp turns these days, let alone a sports car).
Next, graphics. Yes it's a sand box game, and the characters do look impressive, but the city's beauty is so varied it's scary, areas that are open and have a large view are impressive, but some confined areas are downright ugly. For a game that expects it's players to roam the entire city, a set standard would have been nice.
3rd: Camera, whom ever said the camera is fine because it gives totally control has obviously not had much experience with action games. The camera is usually good but in certain instances (particularly indoors or while driving) the angle can be completely useless, but considering it's something that plagues most action adventure and sandbox games, I'll let it slide.
4th: Multiplayer. While teamdeath match is pretty cool and each mode has it's own little things that make them great, they just aren't balanced. Weapons are horribly balanced to the point where a person who pulls rocketlauncher is almost always a winner. I don't mind it that rifle > pistol but when an pistol can NEVER win against an assault rifle, that sucks.
Game modes, I've only 2 that I feel are utter trash, and they are the 2 that are unique to GTA. First one I can't remember the name of, but the head crime boss gives both teams the same mission objective and who ever completes it gets $ and most $ wins. This game seems like fun, been when you're thrown into the gigantic city, it can be a pain in the ass to respawn at the opposite end and making your way back into the action. It's not a big issue in the other game modes because when your objectives are to kill the other team you can do what ever you like, but when there's a certain point you have to get to, it's a pain in the ass. Next one is the cops vs crooks. Great concept and I hope future sandbox games and GTA's could use this. My only issue with this one, the world map is TOO FRIGGIN HUGE. It's great that they've made such an expansive world, but it makes this game mode broken. If you're the crime boss, get on a motor bike and you're basically home free. If they forced smaller maps for this mode it would have been awesome (because it'd force a confrontation between cops and crooks) but I've played twice as the boss and once as the cop and both times as the boss I made it the getaway vehicle with no detours and without contact with the opposition team. As the cop, even on the sports car I couldn't keep up with the tight cornering of the motorbike, also didn't help that I died once and respawned on the opposite side of the city.
It may sound like I hate the game, I definitely do not, I'm actually enjoying it alot. It's the first GTA game that I find myself actually liking. While it deserves alot of praise and the R* deserve many kudos for their achievement and it's definitely a great step forward for the franchise, but it is far from a 10/10 game.