I think the missions are pretty much more of the same but for me, the better graphics and the stunning attention to detail puts it ahead. The 'cover' system is pretty good too.
Theres not as much 'dead space' between the playing areas as in San Andreas and sometimes I find myself strolling through Liberty City just looking and listening to the sights and sounds of a fleshed out playing area.
The different radio stations are heard as cars go past; jostle a smoker and he'll drop his cigarette and it'll roll along the sidewalk into the gutter; drivers get out of their cars and will fight in the street if they have accidents ... there's so much in there to admire. I even spotted a postman collecting letters from a mailbox the other day and that was after over 12 months of play!
And when everything is completed there's also the video editing facility with the PC version.
GTA IV and Flight Sim are pretty much all I play at the moment, as the mood takes me.
Alex