Not so sure. In L4D1, as a hunter, it sort of seems if there's a bit of roof that you logically feel you should be able to jump upto/onto, you can.
In L4D2, quite often there's invisible walls you jump/walk into.
Have you been playing vanilla on official servers or the promap versions that run on most 3rd party servers?
Vanilla has lots of obvious places blocked.
Just look at CC for an obvious example.