My suggestion pertains to the diplomacy screen. In the beta right now, diplomacy is done by opening your diplomacy screen, clicking on the nation you'd like to talk to, which opens another screen, where you can now make proposals and have them accepted or rejected. This is a pretty standard system, similar to the likes of the Total War series, Civilization, GalCiv2, MOM, etc.
There are a number of problems with this system.
(a) Trading for what you want requires a guessing game of what bids the computer will accept or reject. A typical trading strategy consists of making a bunch of rejected proposals until the computer finally accepts your offer. This game mechanic isn't fun, so why not streamline it? [Sidenote: you don't actually have to make proposals in Civ 4, the computer will tell you whether it's a good or bad offer before you make it, but you still have to put the offer together before you know, which is a waste of time.] I'm eventually going to find out how much the computer values my tech/treaty after it rejects a bunch of proposals, so why not just tell me up front that it values my tech at 500gp, or that they're unwilling to trade altogether due to our bad relations? It sure would save me a lot of time and frustration.
( b ) If I want to find what is available for trade, I have to go through and click each civilization before I know. Why not just tell me who has what in one place? And furthermore, why not just let me trade with everyone on one screen instead of making me trade on a bunch of different screens? Again, it would save me a lot of time and frustration.
(c) Further to ( b ), not only do I have to click on every single Civ to see what they have, I have to do it nearly every turn, in case they have anything new. This would partially be alleviated by my above suggestion, but it would also be great if I could choose to be notified when someone is willing to trade me a tech I don't have, or now willing to buy a tech I have to offer, or willing to do a trade pact, etc. It doesn't have an annoying pop-up, it could be a small tab attached to the turn summary screen.
Well, that's all I've got. Feel free to post any other suggestions you have for the diplomacy interface.