The user interface is a very important aspect of any game. UI technically encompasses any system that allows the player to influence the game and vice-versa. For example how the controller maps to game actions, the Heads Up Display (HUD) that gives the player status information and even the audio system. Lately I've been concentrating on prototyping several of these features, but mainly, some of the more interesting HUD elements. From a game design perspective you don't want to have a HUD that's too cluttered. Presenting too much information can often be as bad as presenting too little information. A HUD that contains a lot of different elements that stay on screen can be extremely confusing to players first picking up the controller. This means that a large part of prototyping these features is trying to find the magic sauce to make everything work together without completely alienating new players.
One of the most interesting types of UI elements is the pie menu. I think this is an underutilized technique and one that new players can quickly understand. Ratchet & Clank Future used this technique well for choosing weapons. I have other ideas for it which I'll be talking about more in the future. So far it really seems to be working out pretty well for exposing some more complex options to the player which are quickly selectable.
http://www.piemenus.com/ has quite a few articles and pie menu examples if you are interested in learning more.