I think one of the biggest faults in every GTA game was that each game had a lot of unused, 'dead' space, meaning there were areas that served no purpose other than purely for looks. It would be great if say, more places offered something interesting, a purpose. One example is the wharf/pier in GTA IV. It looked like a big shopping center, but it didnt have any restaurants or shops that Niko could use. Even perhaps the amusement park was a letdown for me. When screenshots of the coaster were introduced, me, like a lot of people, assumed it was rideable or held some gameplay purpose. Maybe on a date, you played a few carnival minigames to win your girlfriend a prize. I like those little gameplay elements in GTA games. GTA IV lacked majorly with that stuff.
Specs: AMD Phenom II 940 3ghz, ATI Radeon HD 5870, 4gb DDR2 800, Windows 7 Professional x64