From some preliminary observations on the Disgaea 4 board, it looks like the platinum trophy is going to be harder than Disgaea 3. There are trophies for collecting ALL items (rarity doesn't matter, but I'm sure there will be hard to acquire items like from bosses) and viewing all skills, and anyone who has played Disgaea 3 will know that getting all of the skills will take a good while because of the hidden/special skills.