(Eh. You can improve your odds a lot by having a more reasonable set of rules - then just about any game can be fun, for the same reasons non-diplo games are fun.)
I played in a Game of Thrones diplomacy game once. It was quite enjoyable in a lot of ways, as it turned out (largely due to not having to worry about "declaration" rules).
However, at the start of the game they explained that so-and-so was King and we were all his vassals. We were all expected to swear fealty and then to discuss the state of affairs and who might be chosen for Master of Coin (a meaningless title with no actual powers). Finally, some people got tired of it or got booted and some things started to happen.
Where is the fun in that?
Were I to make that, no way would that fly. No, set the game at the end of the first season/book.
Like so:
* The King has just been killed, and Ned Stark has been beheaded by the "new" King (Joffrey).
* The King's two brothers both have a claim to the throne (Renly and Stannis).
* The Northern King wants revenge (Robb Stark), and has a large army behind him willing to put him on the throne.
* The Iron Islanders are at war with the North.
* In the East, Daenerys is raising an army and many back her for the throne as well.
* Meanwhile, the White Walkers are coming, adding pressure to the whole situation. (Ideally an AI with incredible numbers of armies.)
Now THAT would be an interesting diplomacy.
Give people motives and things to fight over - that's what diplomacy is all about.
@M. Poireau Indeed, although I am not really a fan of complicated backround stories. These already scare of many potential players. I found my own solution with th e Ideology game series. The game takes place in a 20th century scenario - either in the 20s/30s or during the Cold war, while the political ideologies determine the player's possibilities. For example, fascists and communists could break some rules. The last ideology remaining wins. By that the players automatically start conflicts and even roleplaying.
At the current stage of developement I need 3 normal rules, one or two sentences for each ideology (their number depends on the historical era) and 2 "special case" rules - for example to prevent people from "gifting away" their border to neutral players when war is declared.
In the end it always depends on the players. If they really WANT to play the scenario as it was supposed by the rules, they will always follow them.