Wow.
A puzzle game not for the faint of heart.
It's almost as if I'm actually untangling a humongous knot. You've managed to capture the same kind of frustration, but made it in a fun interactive way. The difficulty curve on this one is pretty steep, so to those who aren't up for a good challenge, don't bother. I have no qualms with that because the concept is simple and it's easy to learn. So once it hits that tough climb, it really makes you think instead of just randomly moving things.
But then again, I do suspect the difficulty curve might put off some 'casual' gamers. What I would suggest there is to create level packs with different difficulties. Perhaps a set of levels that are slightly easier, and some that are slightly harder. Here once you start hitting level 4 or 5 it starts getting really tough so the more 'casual' gamer gets a fun 3 levels and then starts getting frustrated easily.
I do think I've actually played the old Untangle before, and I must say this is a vast improvement from the previous game. The graphics have gone up several notches and the addition of a story is good enough. Nothing complicated needed. We are untangling string anyway, how complicated can that be?
You should look into creating a level designer. I think people who enjoy this game will certainly look to creating levels.
The challenges concept was a great addition. Replay value added nicely there.
A pretty much gorgeous interface, simple gameplay and challenging game. I haven't had the time to finish it yet, but I think I'll definitely attempt it. If there was a medal for finishing the game, it's one I'd be proud to get.