I truly love "short" games, especially those that are "easy to learn, somewhat challenging to finish, and difficult to master" like Mirror's Edge, Braid, Ikaruga, etc...
Like, ME, it's possible to finish it in 8-10 hours if you don't do time trials.
Braid, it's possible to spend a little bit of time playing, and then the rest of your time thinking about the puzzles that you're stuck on in other places during the day, come home and work on it, and finish the game in 2-3 hours. Then the game gives you speed runs and for those clever enough to discover them, the stars to collect, which are a huge timesink (Even if you cheat and look up where they are, one of them takes 90-100 minutes to get, and most of the others require extremely difficult and skillful play.)
Ikaruga, you can play on easy and do relatively fine - while it's difficult, it's not impossible...But S or S+ ranking stages on hard is incredibly hard.
Portal, like these other games, I found to be a refreshing, mildly unique, fun experience. And I'm glad it didn't go on much longer, because people again, say it's "short" but there's the advanced test chambers and again, the games support for speed runs and downloaded maps.
Games like this are great - they are fun, unique, and the experience starts to get "old" just before you beat it. Imagine if Mirror's Edge dragged on for 10 more hours? I'd prefer less content that is perfected than more content that was rushed and unpolished.