Pulsipher Game Design

I used to design lots of Diplomacy variants, many of which are available through the Variant Bank. While I don't play any more, I occasionally use a mechanism that I first encountered in Diplomacy in one of my designs.

It occurred to me to ask myself, what characterizes Dip, what is the essence of the game Diplomacy: what makes someone look at a game and say "that's a variant of Diplomacy".

The list of the "essence" is pretty short. Diplomacy and its variants seem to share the following:
Always, simultaneous movement (but some people call Game of Thrones:the Boardgame a Dipvariant, and it isn't exactly simultaneous movement).
Always, the support mechanism.
Usually, centers maintain units in a zero-sum fashion--and while some games give economic points to spend in various ways, players still must pay maintenance for existing units.
Usually, no-holds-barred negotiation.
Usually, an area board and one unit per area.

What could be added to this list?