I didn't realize that completing the main Portal game unlocked extra levels, until p3lb0x told me so
It's indeed a very nice little game, with some great humor. And the way the story progresses is nice as well...
I love it that you start in media res, in a clinical environment, without any clue what's going on... I really thought the game would end with the incinerator, but nooo - it's like it doesn't start until then. Lovely!
It's funny that portal technology (used as an engine technique) is pretty old, but it took until Prey and Portal to incorporate it as game elements (as far as I know, anyway). It's too bad Valve didn't go all the way with this, though - portals have to be fired at planar surfaces (ie, not on a hilltop or anything "funky"), regular HL2 AI doesn't take it into account, etc... I'd love to see this being used in more games, although I can imagine it's very hard to incorporate it in non-puzzle games, since it would make "cheating" so much easier.