One word: fun. The GTA games used to be the epitomization of sandbox joy - GTA 3, Vice City, and San Andreas are some of the most entertaining games I've ever played, and yet GTA IV felt more like some sort of gritty crime drama; in other words, it was fantastic, and had it not been part of the GTA series, I would have judged it differently. But the fact was it was from a game series that had been built around cartoon-y antics and humour that gave the previous games an incredibly light-hearted and almost playful feel, despite the somewhat serious tones of the story (in San Andreas, anyway).
GTA IV took all of that away. There were no rampages, the cars handled like shit, the whole "take out your friends" thing got annoying as hell after a while, there were no awesome jets that fired rockets - it was just trying WAY too hard to be gritty. Even in San Andreas when you're planning to rob a casino, or doing jobs for a secret government organization, there was always this humourous, colourful feel to the game - your reaction wasn't "okay, time to rob a casion", it was more along the lines "YAY TIME TO ROB A CASINO WHEEE". I was amazed how a game with such spectacular graphics and such incredible game design could FEEL so bland and monotonous. When I think of Vice City or San Andreas, I'm reminded of the colourful cities brimming with light and energy. Even when I think of Liberty City circa GTA 3, the game still feels lush and fun.
When I think of GTA IV, I imagine a brown, depressing cesspool of a city filled with harsh realities and corrupt, greedy people that are going to be giving you a fucking hard time at some point in the game. The thing is, EVERY GTA game was like that - GTA IV was the only one to actually FEEL like that. THAT is what's wrong with GTA IV (and after all that, would you be surprised that I actually LIKED the game lol)