I've known for a while that some factions have secret diplomacy bents, but I've never really stopped to to find out what all of them are.  It makes sense to assume the devs might have implemeted one for each faction.

There is a chance that this isn't per faction per se, but is actually secret effects tied to the personality trait.  I'll give an example below, but I don't think this is the case.

By the way, if there is a thread or site that already accomplishes this, please feel free to post it.  I haven't found it, yet.

Sophons

They like when the player builds a small military. This could be tied to the "Pacifist" personality trait, however I've never seen the Unfallen become happy when I build few ships.

United Empire

They like when you are militarily strong, although I don't know what the specifics are. If it's having a strong military presence, partaking in combat frequently, or winning battles.  But there is something about military that makes them like you.

Everyone Else

I have no idea! That's why I started this thread :) It's possible that many of them don't have specific nuances. For example, the Unfallen have a very high pacifist rating in general, so maybe they don't have any special diplomatic mechanics?

Also, it's possible that these diplomatic messages sent by these factions don't represent anything game-mechanic-wise, but from my experience, I believe they do.

It's more like "they like you when you're losing and they hate you when you're winning."  In my experience the AI takes no prisoners when it knows it's winning and is absolutely shameless about begging for help when it's losing.  The lowest AI on the scoreboard is always the first to beg for an alliance and the highest is always the one to bully everyone around.  Every diplomatic deal they offer they either seem to ask something ridiculous in exchange (like, PLEASE truce us...oh and give us three techs for the truce too!  PLEASE?) or gives you something completely random and possibly useless (here's some bluecap mold when you've set your system development to use jadonyx and have no minor pops that can be boosted with it!)  Trying to MAKE a deal with them is an exercise in futility; the AI has its own set of values for every resource and you're not likely to come away with a deal that benefits you even relatively equally.  Even when you accept an alliance with the weakest link, it usually means you're instantly at war whether you wanted to be or not and this loser AI WILL screw with your war state and make deals and offer surrenders and tribute that YOU have to pay, all without your consent.

The bottom line is this: the AI is not your friend.  Even when it IS your friend it is trying to make you lose.  You're not fighting 7-11 other AI's; you're fighting ONE AI that's trying to mess with you from 7-11 different directions.  I've been seeing it ever since Monopoly got a videogame adaptation way back on the NES; even when the computer is split into multiple parts, they're all still against YOU.

Your ideas aren't necessarily incorrect, but you've missed the point of my post, I'm afraid.  

There is a feature in the game where certain AI factions' disposition to you will increase based on specific actions that you take. (You can read the OP for examples.) The purpose of my post is to figure out what those player actions are. If you aren't interested in figuring that out based on what your opinions about game AI behavior, that's fine of course.

Diplomacy status click on empire info. Bottom right corner 'personality'.

Each faction has a random list of personality traits they can get