Personally I think it's more about how linear a game feels rather than how linear it actually is. HL1 and HL2 are really no more or less linear than the one another, but HL2 sets itself in what should huge open environments, intersecting city streets, beaches, etc. You are constantly running into both literal and figurative road blocks to keep you on your narrow single path. With HL1, when you're spending the bulk of your game experience in some underground research facility, the lack of freedom doesn't exactly pull you out of the experience.