This is the second thing that makes World of Goo so unspeakably special. It's not a one-trick pony. If it were a pony - and we all know it's not - it'd be up some stilts, juggling chainsaws and pulling the flags of the world out of a hat made from rainbows. There's a constant introduction of new goos, and intelligently designed new ways to use the goo that you've got. No level, in any chapter, feels like you're needlessly retreading old ground for the sake of making the game longer. As a result, it's not the longest game in the world - but you'll love every minute.
So, the beauty of the game is just one piece of the jigsaw - the sense of constant accomplishment and awe at the new things you're being allowed to do is the other. It's only a two-piece jigsaw - it's amazing that so many huge development teams filter their ideas through dozens of committees, and spend millions of pounds getting it so wrong.
The progress is constant, because the basic objectives aren't too difficult. The levels can be challenging, but they present themselves in a way that gently nudges you towards the solution. And if you make a mistake, your moves create time bugs, which you can click to undo a move. Unlike, say, ooh, off the top of my head, Braid, there's no sense that you're wrestling with someone who's more keen to show off than keep you interested. But that's not to say that 2D Boy doesn't have the inclination to mess with your peace of mind.
Every level has an OCD criterion for proper completion. This might be to rescue a certain number of balls (limiting the amount you can use in your construction, or lose to the environment), or it might be a harsh time or move limit. Getting the OCD requirements gives the game a valuable replay value for hardcore types - more casual players will probably be happy playing through it again because the basic goo-building process is so enjoyable.
Which brings us to the final area - the World of of Goo Corporation. Every goo you rescue beyond the level's basic requirements gets despatched to this tower-building sandbox. It's a homage to the game's conception - a game prototype called Tower of Goo that's still popular on the Experimental Gameplay Project website. But it's also a brilliant psychological trick to get you going back to all the levels; the height of your tower in the Corporation is seen by other people as a cloud in their skyline. And you see other people's clouds. So you go back to all the levels, foraging for spare goo like some kind of oily hobo.
Let's mention a couple of bad things, for the sake of bloody-minded balance. Speed can be important on some levels, and if there's more than one goo crawling around, it can be difficult getting the one you want at speed. Other people will say it's too short, and compare the hours spent playing the game against the price, with a miserly tut. If you do that; if you've posted anywhere, on any forum, in any comments thread, that "it's a bit expensive for what it is", then please do me the service of punching yourself in the face.
If there's any justice, World of Goo will set the internet aflame and earn its creators a million dollars. The members of 2D Boy should be carried around in chariots while this glory lasts - because at some point, they're going to realise what a murderously high standard they've set for themselves, and run away forever.
World of Goo is due out imminently, and you can find out more - and pre-order - on 2D Boy's website. A separate WiiWare version is also out soon.