I imagine this to be some kind of gamemode or only used in a level or two orso. An entire game ? Must be insane
The thing you must remember about Portal, is that the entire game is a series of rooms. Each room contains one puzzle. Once you beat one puzzle, you proceed to the next room, where another puzzle awaits you, albeit a more difficult one.
That being said, I have high hopes for Portal. The concept behind it is innovative, but it's not a game you can say "it'll be interesting to find out what kind of applications it will have in the game", because it IS the game. It is like calling the ability to drop blocks in Tetris a game-altering ability: without this feature, the game would not exist.
Valve got a lot of feedback about the HL2:E1 episodic content, and I think a good percentage of the playerbase said that, for $20USD, 4 or 5 hours of extra Half-Life 2 gameplay was worth it, but only just. Valve is making up for it now, by releasing both HL2:E2 (which is rumored to be twice as long as E1), Portal (an experimental single-player FPS based on the Source engine), AND Team Fortress 2 (the long-awaited sequel to Team Fortress {Classic}), in the same package, for the same price as HL2:E1.
[HL2:E2, Portal, and TF2] for $20. Now that is a deal.