Its Grand Theft Auto without the cheese and grandeur of its predecessors...
Personally throughout my time playing this game, I was bored the hell of going through the sloppy dialogues; conversations which gets you to nowhere and sub-par voice acting. If anything that annoys me much is the constant rantings made by Niko in his native language to stress that he is from Eastern Europe. The plot is very inconsistent and difficult to follow; it often switches in-between from the protagonist seeking his fortune to taking revenge on a former soldier who betrayed his platoon during a war.
Missions are the usual fetch-and-kill but without the jest you would find from previous GTAs and that is contributed largely by the inconsistent plot line and sub-par script mentioned above. The only mission which thrills me so far is the bank robbery and funeral mission which is damn funny due to your Irish friend, Packie's funny swearing.
Weapons selection are reduced to the normal generic. No flamethrowers. No kantanas. No metal knuckles.
Side-missions are reduced too. No paramedics. No taxi driver. No firefighter.
Acquisition of assets / businesses and safehouses are also eliminated.
Those of you who are rooting for territorial wars between gangsters seen in GTA: SA will be utterly disappointed to see its disappearance.
As for the multiplayer aspect of GTA IV, don't hold your hopes high, you will be ultimately frustrated throughout your time playing it because of the community. (What do you expect from playing with people in a game where you're supposedly required to act as a jerk to win huh?) Hosts can be kicked by participants and there is no lobby for you to browse what 'rooms' are available for you. And I simply lost count the number of times I witnessed hosts getting kicked simply because they don't like the selected game or they are just simply morons.
Good luck to those who wants to grind some achievements on the multiplayer portion of GTA IV. You're in for some ultimate frustration.