when i started it up it was no different than playing a regular game. besides having everything you already have. what am i missing here. i don't want to play the game all over again. but its the only things standing in my way of a platinum trophy. 

I would say that it is amped up on difficulty. I too am working on the platinum. Three trophies to go.

The difficulty increases and you can now upgrade weapons even further. I also believe there a side boss you can fight after the end of the game (pirate robot thing?).

@cedellh: Good luck with the Plat. It wasn't too difficult but I had fun doing it.