I stand by what I have said already,
"What would I have liked to have seen then and what would I like to see now.
I think if the book had been broken into terrains, it would have been stronger, far more useful and have become inspirational.
e.g. Arctic - with rules for cold, a new equipment list (sleds, warm clothes, eye protection, new spells, and monster section (including huskies, polar bears etc).
And so on for desert, forest, plains, mountains
You see as DM, the setting can often fire the imagination. Rules don't do it alone, and I like rules. Wilderness is just too broad a terrain type!
I might want to take my players to Hoth - if so I want rules that deal with this easily accessible, not rules on cold spread throughout the entire guide. After reading these imagined arctic rules, perhaps I may have been inspired to take take my players to Hoth, even if it had never occurred to me before.
Or Tatooine, or up the mountains. Or in the open plains, where horse lords rule.
I also think these guides would have been more effective if sold with a DM screen - all relevant tables must be on the DM screen. How much can one remember otherwise?
My dormant Old School Adventure Guide - of course gives Kim the last laugh. He has published I have not! So, take a bow Kim for the WSG, you at least were an important part of the silver age of D&D