Personally, to me the main draw of a "next gen" GTA game is not the missions or minigames, but just to cruise around the new, detailed environment and listen to the radio channels. I will be doing the missions, but just so I get access to better stuff.
I got pretty much enough of the gameplay aspect with Vice City, San Andreas, Liberty City Stories and the two PSP games, so I'm kinda bored to do those similar missions all over again.
Hopefully GTA4 has something new to offer in gameplay too.
Still, GTA4 is among my top 5 most anticipated games list, GTA's are now more of a "chillout experience" to me than games.