Yeah, diplomacy is definitely interesting. While for better or worse it starts with all players "seeing" other empires via diplomacy screen from the very start of the game (at least from I can tell), this does mean the already mentioned non-standard diplomacy can commence relatively early and at full speed. Personally I think it's great because I like how SR2 handles the diplomacy.
To expand a little on forsaken's post - during the game, you generate influence point by various means, either planet resources, iirc some special buildings etc. You then, in turn, use this points to buy diplomacy cards. A fixed amount of cards can be seen and bought by all players in the diplomacy screen and every 30s a minimum of one new is generated. If no cards are bought in the current interval, the oldest gets scrapped and is replaced by a new one. If more cards are bough, the same amount of new ones are added, but only at the end of interval. The cards that are not scrapped or bough have their IP price reduced with the next interval. So in theory you can get initially very expensive card relatively cheap if you wait a few minutes... only if AI/other player doesn't snatch it before you.
So when you have this cards you can play them. Each card, in addition to having "buying" price, also has "using" price - you have to spend IPs again, to use them. Helps limit the card spam, you see. Anyways, cards range from "name a planet" (which names target planet in your empire and gives it boost in certain areas for few minutes), spying on other empires, to "annex system" which does exactly what it says on the tin. Now, you can't just go around annexing systems on your whims though. See, when someone plays mentioned annex or spy or just about any other card that also interacts with other empires, this count as a "proposal" in galactic senate. Players may either agree or disagree with such proposals, and you do that again with cards, like "negotiate" card and such. You can play as many as such cards on a proposal as you have IPs and actual cards. Depending on a final yes/no score the proposal is either rejected or accepted. So all in all not far from real life senate, hehe.
In the end you get diplomacy that is a kind of a mini-game within a game and requires some actual planing and occasional twitch card buying, far from your standard, static "give me 200 gold for this tech" you get in games like Civ. It also helps mitigate the problem of AI acting like a robot just going along it's scripted "does this player have strong military? If not ATTACKATTACKATTACK" pattern, the problem that plagues oh so many 4x games. This is all veiled behind the senate play, and with diplomacy being seen to everyone, also negates either the feeling of AIs having better deals between each other than towards player AND also gives feeling the player is not a center of all attention. All in all, I like that, I like that a
lot.
This is a thing with SR2 as well as SR1 - this games are full of neat ideas. The guys behind them are not afraid to try something new and different. While both games lack some polish in certain areas, imho this just makes you love them more - like a child that might not be amongst top 10 MIT graduates, but you can see he was given a lot or even more than usual amount of love by it's parents regardless.
But that's just my opinion I got since I recently purchased SR2. Been a while since I payed more than 10€ for game, and an early access to boot (iirc last time I spent big moneys on a game was when Distant Worlds: Universe came out). Not regretting one bit of it though.