To answer both questions, in the end it's mostly rolling up your katamari bigger. But there are variations and objectives. I'd buy it if I were you, it's only 15 after GGC if you can find one at your best buy like me.
Also, I have to agree with you I think we might be over estimating how much is being sold. It's not a GTA or anything in sales but for a niche title, and believe me this defines niche titles, it's a smash. I believe an extra printing was ordered but I can't confirm that. If nothing else, it shows companies that just because a game doesn't have ultra violence doesn't mean it's not going to sell. The sales of this game will make developers to take a chance on others. Look at Phantom Dust, that's a niche title and if that sells well who knows maybe more and more games that are primarly japanese will get released here.