First off, I love diplomacy. I was never a 'card game' buff, but this has added a fun, time wasting, addictive game.
I think the most frustrating part of diplomacy is that the NPC's don't play by the same rules. Often, in fact, most of the time, they play cards that give them 2~3 influence but doesn't give us any inspire, demand, etc.
Its frustrating because a level 50 NPC can beat a lvl 85 diplo just because they have what I call a 'cheater deck'. They beat a good strategy because of better cards.
Although after one or 2 parleys, you can develop a strategy against them, its the intial learning (and subsequent faction hit) that makes it bad.
Sorry for the ramble. Early and not enough coffee yet.
Thanks for listening!