If we are talking diplomacy, one thing that I have always hated.
I hate that your diplomacy strength actually impacts the main deal.
That might seem crazy so let me clarify. To me, good negotiators know the real value of a deal, which is based on their needs and wants. They don't let their emotions influence things too much. So if Deal X is good right now, it should still be a good deal if you love me, or still a good deal if you hate me. Deal-making should be influenced by how badly you need something, not how you feel about me at the time.
Now to me high diplomacy should get you "extra bonuses" that the enemy isn't aware of. This is the side deals, the under the table stuff, the bribing of the assistants, etc etc. From the opponents perspectives, they got a perfectly fair and reasonable deal....its just that you happened to get a little extra in the form of some extra yields or something. The opponent doesn't "lose" these yields, you just get them.
Likewise, a hostile opponent may not want to deal, might shut me for example, but its silly for them to make outrageously unbalanced deals that I am not going to agree to. That's just a waste of time and energy. Either have them not deal with me (which is fine), have them go to everyone else first, but if they are dealing with me have them be fair deals I actually would consider.