From: Distant Worlds
Start a war in order to 'liberate' a planet from your neighbour. Contact your trading partners and sell the galaxy map/technology for some cash. Contact your rather pissed off neighbour and trade some cash for peace once you get/about to get (your guys fighting down there) the planet you wanted. He agrees without giving himself a chance to take it back from you. He is bigger than you and all he has to do is ignore your proposal.... But no..he agrees for some mere money to let it go.... You got a brand new planet, and he is got one less.
In case, if you don't have any maps or tech to trade, there is always an option (measured in currency) to declare an embargo on some far away galactic empire and get the trade you want with the trading partner.. Whether it is peace or some new amazing tech that you're after. Doesn't matter.
Point is, at the moment, diplomacy makes it too easy. Playing on hardest settings, longest research, harshest initial settings, etc. In my humble opinion, it would be much better if a player wasn't allowed to ask for peace right away if his/her empire was the aggressor. For example, a two-year time period that will have to pass in order for the AI to consider it. That way, the gamer may experience some true consequences of greed and arrogance... At the moment, reputation hit just isn't enough.
By the way, for me, personally, shadows is easily the best expansion so far.