It's strange - ordinarily, I feel like a game should ideally last between 20-30 hours. It usually takes a bit of time for the world's details to sink in, and then a little more time to wrap things up while the game is still a preoccupation. Too short, and I can't really get into it; too long and I lose interest. The Portal series, clocking in at 10 hours, should probably feel too short.
And it kind of does. I want more Portal. Navigating 3-dimensional space, solving puzzles, being insulted by GLaDOS - I could do that forever, probably. Yet I don't really feel "cheated." The Portal games are short, but they don't have an ounce of fat on them. There's no padding, no busywork to stretch out the play-time.
I can respect that. As much as I might want double (or even triple) the Portal, I can't say with certainty that it wouldn't have worn out its welcome eventually. The creators must have felt that there wasn't more game to be had, and they didn't want to force the issue. It's kind of a bummer, but at least I got 10 hours of near-flawless gameplay out of the whole thing.
From here, I have a couple of options. I could try and complete the challenge maps, or explore Portal 2's co-op mode, or I could replay the games with the commentary turned on. I think I'll go with the last, primarily because I'm intrigued. I will often watch movies with the commentary track on, but so far I've never encountered a game with this feature.