If you haven’t heard of Portal yet, then you should take gander at Valve’s Orange Box site – specifically, the Portal page, and watch the trailer.
Look mind-bending enough?
The team that built portal was hired because of their work on a little project called Narbacular Drop, in which you play a certain “Princess No-Knees”—aptly named, given her inability to jump—trapped in a sentient dungeon by a demon. In Narbacular Drop, the goal is to escape, and you’re often put in situations where you can’t reach the exit. The magic here is that Wally (the dungeon) wants to help her out to spite his boss, the demon. So, upon command, Wally will open up portals in pairs, one for ‘in’ and one for ‘out’ – essentially, two sides of the same doorway. You could, for instance, place a portal on a wall near you using one mouse button, and another on the furthest wall or even on the ceiling with the other button.
Your momentum is conserved, so you can fling yourself across rooms by falling into one portal and flying out one you’ve placed high in the wall. You can even build momentum by then placing your entrance portal on the floor where you’re about to land. Diagram time!

Now, Portal is roughly the same game as far as I can tell, except much prettier and you are actually able to jump short distances. While I haven’t played the entire game—getting to level 11 or 12 at the time of this writing—I did notice one very interesting omission: the difference in the implementation of the portals themselves.
There’s a fan-made map for Narbacular Drop called Jailbreak. In this level, your physical movement is restricted by the simple fact that you are locked in a jail cell. Escaping via portal is—unfortunately for Princess No-Knees—impossible, since the only portal-friendly surfaces are on the ceiling of her cell and outside it. You see, in Narbacular Drop, you can place an entrance portal on the ceiling, and then an exit on one of the walls outside your cell. You can then look up, and (now looking out the wall), place the exit portal somewhere else, instantly changing your point of view. In effect, you are able to explore the jail without even physically moving.
In Portal, however, this type of malarkey appears to be impossible: any attempt to fire a portal through another portal fails. I’m not sure what sort of testing decided to remove that (assuming it’s actually removed and that they’re not going to upgrade the portal gun in later levels), but it does remove some lateral thinking from the repertoire of puzzle-solving skills.
The levels so far haven’t seemed that difficult, but it’s possible that having played Narbacular Drop, I’ve already gained most of the needed puzzle-solving skills, and am currently just matching A to B in a set of prettier graphics. Since I haven’t gotten through the game, nor any of the bonus levels, I can’t say that I’ve been there, done that quite yet.
Overall, Portal—so far—seems to be a fun (and beautiful) adaptation and extension of already innovative gameplay.