As gamers and game developers we know that immersion is everything. When you're immersed you lose track of time and become involved in what the game is presenting. A major factor in what makes or breaks immersion is how easy it is for your player to convert an idea into an in-game action -- that is, how fluid your game's User Experience (UX) is and how well-designed its User Interface (UI) is. A game hurts itself by providing too little information or too much, requiring too many inputs, confusing the player with unhelpful prompts or making it hard for a new player to interact. Poor UI design can even break your game completely.
Game UI By Example - read the following article:
A good UI tells you what you need to know, and then gets out of the way.
Jakob Nielson is one of the guru's on user interaction
Jakob Nielson's 10 UI heuristics
Using a game you currently play can you identify some good or bad user interaction