Look for yourself, technically it's another world. So much more realism.
And still, great graphics is not the essential factor buying the game for the second time, after I played it for a long time on the PS3.
But there are additions to the gameplay, too. Especially that you can play it as a 1st person shooter. Yepp it's coming. There's a video showing the first person mode, too. This makes it a whole new experience to play. Even though I know it's not the same playing GTA from 1st person view, I think it will be awesome.
And there is another tiny reason. It's GTA! And I want this on my new console, too. The PS4 is not backward compatible. And only a few games are interesting for me.
I'll wait to buy a PS4 until Uncharted 4 is released. Together with GTA V / PS4, I'll have reason to turn on the console and don't feel sad to spend a lot of money. Yes, GTA is that important for me. And I'm pretty sure there are more people with the same thinking.
I hate to admit it, as I was infuriated that Rockstar didn't release GTA V for the PS4 first, instead using the last gen PS3, but the company did the right thing.
I bought a new PS3 for GTA V (my old one was damaged 2 years before), and I will buy GTA V on the PS4, too. It has such an impact.