I have finished Alternative mode and haven't tried Normal mode.I liked the Alternative mode. The challenge was well ballanced and appropriately difficult for me. I wouldn't want the game to teach me how to solve the puzzles (which is what I expect it does in the Normal mode to an extent). It's true that I'm not planning to play the Normal mode (nor replay the Alternative mode). Or is there a reason why I should play (or have played) Normal, other than extending the gameplay time?
I like Nifflas' level design in games he released before Knytt - he knows how to make a solid "video game" challenge.
I found NightSky (in the Alternative mode) too long though. Maybe it's because the setting
1, the minimalistic form and the story
2 didn't particularly appeal to me. The puzzles and the physics are really good on the other hand.
I really liked the mood of Aurora's North 14. It was a very pleasant surprise when I (as the hero of the game) came into a city after hours of wandering in nature. Too bad there were only three trivial screens of it. I think I would love the game if all of it was set in a city.
Maybe it was just the lights though - I really like city lights. Without them, levels in a city would probably be similar to Perpetuum Factory.
I was disappointed by the anticlimactic nature of the story.