I agree with Portal. Without the advanced and bonus challenges, that game is terribly short. I'm glad I picked it up off Steam when they were offering it free last year . . . I woulda been pissed if I had paid for such a short breeze through.
There's been an ongoing trend recently with short FPSes, though. I played through Modern Warfare 2 on veteran in a matter of a day (maybe about 6-10 hours, don't remember clearly) . . . and I'm not going to bring up Black Ops.
TBH, I think it has more to do with the mulit-platform development, console titles are known for being shorter in general compared to PC titles. The length of these games is fine on console, but comes across as being too short on PC, leaving us feeling jipped. Maybe it wouldn't be that bad if we got a little extra loving out of it - bonus maps and/or challenges, extended campaigns, etc. Devs used to do this kinda stuff when porting a title to PC . . . but not anymore.
Oh, well . . .