Kicking cow. Sounds stupid? Well. That's because it is stupid. But... I didn't really get excited about GTA V until I saw that. Wasn't even going to pick it up, and that sort of stupid stuff just sells a game for me. Little details like these, in such a huge game - lovely, isn't it?
Aside from that, as others have said already: The map. I loved GTA III, Vice City and GTA IV, but offered what felt more like a world than a city. I've always missed the rural areas ever since. Those, in combination with the actual city, made Los Santos feel both huge and diverse. While you could potentially get lost in Liberty City, it never felt as huge or as much like a real place as Los Santos did, to me.