I wasn't really a fan of this approach though. If 8h of the game are almost completely devoid of challenge, another 2~3h of me looking for challenging stuff to do won't make my experience any better. I'd have prefered if they had somehow made those challenging parts somehow mandatory but the way the world was set-up contributed to a flat challenge "curve". I might go back one day and get those 100 or so I'm missing. Maybe.
Collecting all the light orbs was awesome, and it was really challenging to get to all of them. Many were unique platforming mini-puzzles to figure out. That's part of where the difficulty was in this game.
There's also an achievement for beating the game with Elika saving you from death only a few times, which isn't insultingly easy. People just ignored it.
As for the achievement, I played in on PC so they didn't even give me the chance to actively ignore it :P