Grittier and more engaging
First, I'll have to admit that i haven't completed a previous GTA game since it was 2d and birds eye view (GTA London). it may have been because, when it went 3d, i never found the story or characters engaging enough and ended up just using loads of cheats and killing as many coppers as i could, but GTA IV has changed that.
GTA IV has a believable story line with incredible characters, with excellent voice acting and script which kept me engaged throughout with its plot twists etc. Also, some of the activities you can do on the side are enjoyable, like gettin p*ssed and trying to drive yor friends home again but bowling, darts and pool are allright to play once but i never played them again afterwards. I have to say that the graphics are an improvement from earlier games, but they aren't revolutionary and i wasn't blown away by them. Gameplay is excellent, i like that they have added a cover mechanic and that when you befriend someone you get bonuses like a free taxi, or some guys to give you backup in a firefight which is always helpful.
To sum up, graphics were a good enough improvement, but this game is about more of an engaging plot and improved gameplay than visuals.