I enjoyed GTA IV, but I agree that San Andreas had it pretty much perfect, as did Vice City.
What made San Andreas sooo good, was the huge map which had everything from mountains, farms and deserts to sky scrapers. It was nairly perfect, and if it had the same level of graphics as GTA IV, it would be outstanding. What made GTA IV a little boring, was the fact the city was all there was, just a huge city. There wasn't enough you could do (that being said I've played hundreds of hours on it still...). I want them to bring back the aeroplane and jet packs and custom cars, the stuff you could mess around with well after the story is over. I used to get gang members to join me in Helicopters and jump out of them for some boss free falling into the ocean. There's just not the same level of fun like that.
GTA: Vice City was all about the atmosphere. It was great for different reasons than San Andreas (best radio soundtrack of any game by far, although San Adreas was very good), and I loved how you had your own goons.
Can't wait for the trailer. Hope they include more characters from the original series, as the continuity was one of the rewarding features of the series before GTA IV.