I agree, GTA IV does have some flaws. I think they have made it too realistic imo. I liked it in Vice City, where the cops were pretty awful, whereas in IV the cops are on every street corner, and if you fire one bullet they are after you. Most of the time in the game, mission or not, you are being chased by the police. However, I do like the driving in IV as it is more realistic and gives you an insight to younger players to find out what driving can actually be like with drifting.
I personally like the story line. It is not my favourte storyline at all, but it is very good and well thought out. my favourite story line is the Vice City story.
I think another flaw is the lack of customization. In San Andreas, you could basically create you own character with a variety of clothing shops, barbers and of course, the tattoo parlour. In GTA IV you only get 2(?) shops, Modo and Perseus (sp?), and you get no barber shops, so you cannot change your hairstyle, and you cannot give Niko some pretty hardcore tattoo unlike San Andreas. I think this is where I prefer Saints Row, because I like to customize my player and make it unique.
Furthermore, I really do not like Liberty City, I didn't like it in GTA III and I still do not like Liberty. I think it's very dark and gloomy, whereas Vice City is just beautiful with clear blue sparkly seas and the weather is gorgeous, whereas in Liberty it tends to rain a lot and when I play it, I can't get myself into the missions playing the game with the city. The city is smaller than San Andreas, which in a way, is a good thing, because San Andreas was just huge. Vice City is maybe a tad too small and would be better if it was bigger.
I like how you still have the ability to swim in this game, but R* have taken away the abhility of swimming underwater, which I think was a good add to San Andreas, as I think the previous GTA's had annoyed gamers that their character (Vercetti, Claude) could not swim at all, and as soon as they fell in the water, they were always sleeping with the fish.
Also in the previous GTA's you took over the city. For example, Tommy takes over Vice City, with his gang being the biggest in Vice City, In San Andreas, CJ can defeat every other gang in Los Santos and the whole gang turf can be his, and he also helps the Triads take over the gambling business at Las Venturas.
Despite these flaws, GTA IV has the best graphics ever seen in a GTA game, maybe the best graphics ever seen in a game. However, despite all the negativity I have pointed out, GTA IV is still ONE HELL OF A GAME!!!