I thought the game was enjoyable for the short time I played through it. It was definitely something different and fun while I was playing it, but in retrospect I don't think I'll be plaything through it again any time soon.

I thought Portal was an excellent game. Sure, it was a bit on the short side, but I didn't mind that too much. On the day I bought The Orange Box on my way home from work, I played it through to the end. And the next day, when I got home from work, I played it through to the end again.

You bet your ass.

I'd have paid full price for it alone and been happy. In fact I basically did, as I already had HL2 and TF2 isn't interesting to me. The gameplay was highly original. The length is perfect.

Perfect? I have a total of about five hours on record and that's a full play through and probably another hour of just messing around. Maybe I'm cheaper than most but assuming normal/regular price is $20 U.S. I expect more than five hours or so out of a game. I've got 50 from Plants vs. Zombies at $10, 50+ and counting from Torchlight, Defense Grid, Killing Floor, I could go on and on. Maybe its adapting sensibilities -- I laughed when a friend told me the single player component of MW2 was only about ten hours of game play.

Length != Quality or Enjoyment. (or so I read in some women's magazine...)
I thought it's length was perfect. and pretty much every review I read of it at the time also thought it's length was perfect. I guess they could have padded the length out with filler, or made you redo some of the levels with slight changes, but that wouldn't have improved the game, it would have made it worse.
Way too many games that could be great are padded out with annoying time-wasting sections. Eg: Okami. A director's cut of that game that cut out a few hours would have been stunning.
Anyway, in my case, I played it twice, plus tried all the challenges... so I spent well over 10 hours with it, probably 15. I'll take that over a repetitive 80hr RPG anyday.
I just got Mirror's Edge in the Steam sale, and it has a very similar feel to portal. And a similar length. But unlike portal it didn't stay true to itself, and added un-needed combat and annoyances. Shame.