Why can't you threaten a weak race with the prospect of war, and why can't you be directly threatened with war? The Drengin and Korath bully me for cash quite often, but when I refuse (which I do every time) they seldom actually declare war on me. I'm not particularly complaining about this, as war is very very bad, but I do feel not being given the direct choice between paying up or them instantly killing you as a slight deficiency in the game. Obviously, if every time you refused to give cash they declared war right away most game wouldn't last long.
The flavour text in the trade box that's meant to be your negotiation is always 'oh my, we're a bit strapped here, chaps, um... you wouldn't mind, would you?' (at least on neutral, which I play most), and you can never bargain directly with 'ok, give us x or we'll smash your faces in, yeah?'. I also do not appear to see how military strength contributes to your bargaining position, and an AI will drive an extremely harsh deal with you even when they should be very aware that in 5 turns' time they could be on the receving end of your entire military force. An AI with an occupied military will also seldom take this into account, I have seen, when you offer them a deal they shouldn't refuse.
The major issue here is probably the diplomacy system that plots against the player in general, where even the Torians, with zero military, will demand that you fork over your entire treasury and most advanced techs for Barren World Colonisation. What they don't realise is you're using your treasury to support your massive navy, with no shortage of [advanced] troop transports, and the only thing barring access to a large quantity of bountiful worlds to colonise is their soft fishy(?) flesh. You'll probably get the tech you wanted from invading their worlds anyway, and a few more besides.
As a player I will take into account when dealing with the other races what my military is doing and how substantial it is compared to theirs. The AI never seems to do this and gives me lip totally regardless of my ability to take said lip and eat it with a salad garnish.
In this topic, we discuss the above.