Actually, this is a really good question
Well, to me, GTA is all about gameplay. Nothing fancy, any gimmicks or unwanted features that don't enhance the gameplay. Just pure GTA that we all know and love. For example, TC:NYC hyped up the game about famous landmarks, and when I played it, the landmarks were either meh to me or improperly re-created.
What's GTA to me?
Everything. From the sound effects, cars, unique sandbox styled gameplay that just cannot be immitated, pause menus, textures, poking fun at things, Jackass Thompson, radio stations, Lazlow etc all of these make GTA, GTA. And hopefully, IV will maintain these features.