I played all the GTA's from 3 till IV and IV is probably my favorite of the series so far. IV took out some of the stuff that worked well for the series, like the in depth customization and added in great physics, interesting mini-games and awesome graphics. The story was just as good, because the story fit the game and it worked well being dark because that was the theme of the game when it came to the story.
As long as they add new things, or improve on a few old things..they don't have to include and improve upon everything the games before it had, to be good. When it comes to things like this, I seem to be in the party that always enjoys the newest game because I don't expect it to be like the games before it.