I've been thinking alot about the UI setup, and would it help if the interface would "build" itself as the game progressed? Take all the buttons that arn't applicable until later in the game and hide them (UnitedPlanets, Trade, even the ForeignRelations button on the main screen.) Then, as the player gets to the point where each button is needed, a popup will alert them to its presence, its purpose, and the option of going to the respective screen. This way, the user isn't overwhelmed with options from the start.
Also, to add onto Myros' example...a while back (about 6 months) I showed my younger brother the game. He's a fan of the RTS, but had never played a turn based strategy game. I won't lie to you, he was quite confused as to what was going on. But since then, we've added tooltips, cleaned the interface visually, and added some screens. Then, last week I came home to fund he had been playing for about 3 hours. He had already won his first game on easy and had bumped up the difficulty for another round...without me sitting over his shoulder and telling him what to do.
peter: If there were 3 main things about the UI that you would change, what would those be. And be specific - "Move these buttons to here because...." or "Sharpen this image and add a box around it." helps much more than "Make the interface easier to understand". Eveybody else can chime in too...the more feedback, the better the game will be!