World of goo is a puzzle game about stacking "blobs?" all the way up into a tube. While this sounds like a terrible idea in theory, 2d boy pulled it off very well.
World of goo starts off easy. VERY easy. The normal missions consist of stacking the gooballs in various shapes (such as bridges and towers) to reach your goal.
However, for completionists world of goo's novelty begins to rot very quickly. With every level, included is an OCD, which is the game's way of saying EXTRA GOAL. This usually consists of saving more gooballs or doing something extra. The OCD system is ridiculous. It is so difficult, it isn't even funny. This is one of the times where you're actually happy to skip past 100% and get on with the game because it is so unforgiving.
In short, world of goo is a fun casual game that is plagued by complex and ridiculous puzzles for the attentive and curious ones. However, I still recommend the game.