Guys, its obvious, we as a part of this community are all mostly used to the Generic classic GTA layout - GTA 3 being the first of its kind, in 3D form, wasnt very much different than GTA VC and SA, graphically speaking. Since most of the people are used to a specific layout, pattern or whatever you may call it, a sudden change of gameplay and realism can shift ones views about the game.
The truth on most part is that most people that say GTA IV was a disappointement, are just suffering a shock, because they didn't get what they expected; If you analyse the situation you can clearly view that if someone is expecting something and don't get what they expect is in no other words, a disappointement. People tend to over dramatize. On my part, if it really matters to you all, think that since this is not what I expected is a good thing. If GTA came back with the same old contexts, and gameplay people would say " Oh its GTA we've seen this before its not surprising " Not to say it's a bad game but were too used to what we've seen in my opinion.
My conclusion is that with the arrival of a new platform (PS3 aswell as 360) GTA needed to redeem its own image towards its customers by implementing something that we haven't seen before, and to me this was the best thing they could have done. I went as far as buying a PS3 to play this game, and I do not regret one penny I've spent: This game is amazing, and we need to appreciate what has been given to us.