Well part of that is the fact that you can play through all powered up. Of course it's going to be easy, try and do it without picking up any of the upgrades.
Haha, that is a terrible suggestion. The game should know that you are upgrading and be challenging the whole way, you shouldn't have to avoid part of the game just so you don't breeze through. That's poor game design.
I hope that one day we'll finally get some of those sneaking levels back from the old games.
I can not stand that in games. Maybe this isn't what you mean, but whats the damn point of leveling up if your enemies just level up with you? I think Oblivion did this and i always hated that. I find that to be a cheap way of making a game difficult.