Atmosphere, Style, and Gameplay all in one.
Gameplay - You play as a small ratmonkey that can run, jump, and climb walls. In the game, you have to use these 3 skills in different ways to get to the item you're trying to collect. That all you do, try to get to the item. The gameworld is incredibly big too. It's not like other platformers where you die if you fall into a pit. If you come across a hole in the ground, jump in it! If you come to what seems like a dead end because of a wall, climb the wall! There are seriously no (noticable) boundaries in the game world.
Sound - I really liked how there is no continuous music in the game. All you hear is ambience. There is random (relaxing) music that adds even more to the game.
Style - I love this game's style. It's so simple! You just explore a vast world looking for little items needed to repair the ship that you were on when it crashed (you were abducted). I was really impressed with tiny details, like villages in the clouds, and little critters that live in every environment, and Metroid style savepoints.
Overall - I HEAVILY recomend this game to anyone. I mean anyone. It's such a charming and addicting game that you'll probably like/love it one way or the other. :D