First, let's just get the visuals out the way. It looks nice and has a nice atmosphere to go with it. I have little in the way the complain about it in that sense. Though since so many other games have used the style since, it's not entirely worth it just for that either.
So it looks pretty and I was extremely excited to play it as I had heard so much about it. But if anything useful came out of this game, it taught me how good puzzle games are made by seeing how bad puzzle games are made. Warned: Some puzzle spoilers
Exhibit 1: Dropping subtle clues
So, in a good puzzle game, there will be a subtle clue that the player can actually notice that will make the player feel a sense of accomplishment when they discover it. I believe that this game's first attempt came early on. You pass by a bear trap in a tree, then two screens over, run into a giant spider. Apparently, getting the spider to attack you several times will knock it down. However, there are several problems here. One, the noise that the bear trap makes as it moves isn't noticeably audible during this. Two, no other bear trap had dropped in a similar way before, there would be no reason to expect it to now. And three, the bear trap was a screen and a half away! And if you died from jumping stupidly once you got the bear trap or from the spider ten times trying to figure out what was going on, have fun.
Exhibit 2: Show a mechanic, then leave it to the player to do the rest.
Portal does this wonderfully, it shows you a concept, then you're off. This game does not. So, there are these brain leeches and the first time I encountered it, I was stuck for twenty minutes until I went online. As soon as it attaches to you, it forces you to walk in a straight line. Ok, obvious enough. The first thing you do is hit sunlight, then immediately turn around. Ok, so I figure that it hates sunlight and standing in it for long enough as it appears to burn would kill it. So I move a block so it would trap me in the sunlight. No, I was just stuck there and after 30 seconds, I figured that's not the solution. There were these dangerous looking things on the ceiling, and earlier when a kid walked by them, he died almost immediately. Ok, so avoid those. So I more or less walked into water and drowned several times until I had to go online and figure out that the things that I thought were going to kill me because earlier in the game that's what it indicated were in fact there to eat the leech off and free me. This was the very first time that you get a leech attached to you, this was the game's big chance to introduce the mechanic then introduce harder puzzles around it. Not only did I get the idea that I should avoid the helpful thing, but it taught the player that the wrong thing would remove the leech. This is poor game design without a doubt, not giving hints is one thing, but giving hints in the exact opposite direction is only for masochists.
Exhibit 3: No clues whatsoever
Even more often than not, puzzles would present with no indication whatsoever. This can be fine, except that something as simple as a destination or direction can be enough. Sometimes I would have absolutely no idea that I could go in a specific direction because it looked blocked off when in fact I was supposed to go there to get a block or something. Even something as simple as an arrow or an enemy whose attack come from off screen could fix this. This is always a silly reason to be stuck in any game, platformer, shooter, RPG, anything.
Exhibit 4: Consistency
Sometimes a jump I was sure wouldn't kill me would and other times, I'd clearly be expected to make a jump that I was sure would kill me but didn't. I never had any idea if any given jump more than two body lengths away would kill me. It makes it rather hard to plan out how to attack a puzzle if the game mechanics itself perform wildly inconsistently. In fact, that's part of why I didn't go towards the left when I should have that one time, I was sure the jump would kill me in the process, thus making me assume it was off limits. I'm sure some of those jumps were meant to be like set piece sort of jumps that never kill you, but it was difficult to tell when those might be about to occur.
In Conclusion: Really, at every turn, as someone who has played plenty of puzzle platformers and adores them, this is simply a terrible game. There is poor design at every turn and absolutely nothing, not even the pretty visuals, can save it (especially not since the black and white shadow platformer became so popular, so it's not even worth it for the aesthetics). Although, if someone wanted to learn more about game design, specifically good vs poor design, this is a decent place to look for answers. There's a definite difference between a game holding your hand too much and one not even bothering to make eye contact or shake your hand every now and then. A good puzzle game should strike a balance inbetween, holding your hand lightly through the mechanism, then giving subtle hints from there on. This game does neither, it holds your hand and lets you trip over a chair then tells you the wrong directions to the grocery store, then seems somehow offended when you get lost on the way there and complain that you're injured.