Light only areas as I understand it are places that are only accessable by using the SMART system.
To add something,
I could have sworn I heard somewhere that there are alleyways that are too narrow for Heavies to fit into.
Aside from being a huge f*** you, the alleyway also calls you fat... can you handle the rage?
Nothing confirmed of course, be it would be interesting to see, especially if Heavies could sidle through said alleyways at a snail's pace.
Furthermore, it would take out a few learning curves to give even a newbie incentive to go lightweight.
[edit:] To SD, I'd like to see a universal ability called "butter" now.