If you think that the dialogue is superb then I feel very sorry for you. GTA4 did not contain a single memorable cutscene for me and all 100 (it felt like 300) or however many there were are just the same old stuff from different angles. No variety whatsoever almost everyone one was in a room with a few camera angles, few characters and several boring lines of dialogue. Perhaps you could quote some of the most memorable as I am sure others would like to know what these are as well.Firstly DONT talk out of your ass about the quality of dialogue, its superb and some of the best there has ever been in any game.
Second, i agree the gameworld is dull and dead once the SP is completed. Just not enough to do. 10/10 it is not but dont make out that its 'awful' because its a helluva long way from awful.