I'm almost through the coop, and the one thing I don't think they're getting enough credit for is the absolute perfection of the difficulty curve. No two tests repeat the exact same techniques for solving them. None are really easy compared to the rest, none are too difficult, and it's never, ever frustrating. Theres no fake difficulty, no puzzles that are hard to solve due to technical issues like the camera or anything. You always get the feeling that you can solve the puzzle at hand if you just keep working at it, and eventually it'll all click. You never really get to that "oh fuck this game" point you get to with many puzzle games. Especially the coop puzzles, juggling two players, four portals and all these gels, bridges and tubes never gets to the point where it's too difficult or frustrating to be fun. Basically, there's no ocarina water temple where it could have been a game full of frustration, and that's just an incredible achievement. Bravo.