JohnSnow said:
Here's the thing. While I agree totally with your theory, calling it "The Planescape Threshold" implies that Planescape straddles the line between the two regions. However, by contrast, I'd argue that Planescape is well over the line into the second region.
Curiously, where on this continuum would you place, say, Dragonlance, Eberron and The Forgotten Realms.
Hmm... Off the top of my head
-5 King Arthur and his knights, the movie Dragonslayer
-4 Howard's Conan, LOTR, Ars Magica
-3 Greyhawk, Warhammer FRP, Palladium
-2 Forgotten Realms, Dragonlance, Everquest
-1 Eberron, World of Warcraft, Dungeon Siege/Asheron's Call, Lankhmar
0 Planescape, Mystara, Kara-Tur, GURPS Yrth, Roma Arcana
+1 Exalted, Talislanta, Rokugan, The Dying Earth, Dark Sun
+2 Earthdawn, Stormbringer, Blue Rose
+3 Elfquest, Bakshe's Wizards, Darksword
+4 Glorantha, Jorune, GURPS Fantasy II
+5 Tekumel, Synnibarr
In general, anything over the threshold has non-tradiditional protagonists. Anything straddling the line has pretty standard character types, but the setting or some of the context might be odd to people unfamiliar with the source. Anything under the threshold can likely be summed by up fighters, rogues, magicians, and kings.
EDIT: Anything straddling the line can usually be summarize by a "high concept" description:
All the D&D worlds, linked by a city of portals where belief is reality.
Chivalric romance meets Greek myth meets swords-and-sorcery.
D&D in the mysterious Orient.
A world where the crusades never ended and magic is real.
A magical world where Rome never fell.