Here's what I finished my first playthrough with:
Finish time: 18:50:51
Max Photo Pts: 5408
Ghost List: 12/33
Unlocked Extra Mode, Mission Mode, Gallery Mode, Hard Mode, Costumes, and an expanded Ghost List. (like I thought, some of the secret ghosts aren't even possible to get on a first playthrough)
When I load my completed file, I get a whole separate menu for the extras I unlocked. There's the option to start a new game plus, do missions, change costumes, etc... Actually I guess I haven't unlocked any costumes yet, just the costumes menu is available.
Oh, I see... the Extras menu option is where you spend photo points to buy costumes and accessories. There's also some new special lenses that I would make a new game plus that much easier (it was already too easy apart from the final boss), and some new camera functions, including a Sense function which I'm guessing would be very useful for finding hidden ghosts.
The missions are just short little ghost battles with pre-defined stats and a score/rank system. It takes a LOT of practice to get even close to A rank on these.