One of the most important aspects of playing a game is feedback. When you press a button, you see something happen, and witnessing the actions that play out on the screen, you learn how to react to situations that crop up in the game.
One of the first types of feedback you get in a game, is moving your character. Whether it is a 2d game or a WASD computer game, one of your first interactions is movement. Left, right, jumping, turning, back and forth. You mouse over an object, and the icon changes to reflect an action that can be taken. You come into range of an enemy and a button lights up. You click on something, and it glows, letting you know that your interaction was successful.
Because of all of this, one of the most important things any game must do, is to ensure that movement in their game is fun, fluid, and fancy (I needed another f word).
One of the biggest issues that repeatedly comes up with video games, is the "underwater level". Adventure games tend to run across this the most, since they usually have different types of levels to help vary the landscape of their games (the mountain level, the ice level, the fire level, etc etc.). Movement in the underwater areas, are usually tedious and un-fun, yet developers have been putting it in for years now, as if their version was the best one ever.
With MMOs in particular, the move important type of feedback is actually the chat box. The biggest complaint I heard from people who liked WAR? The chat box!
My complaint about Champions Online is the chat box. Again, my feedback to talk to others is hindered!
What is the most important aspect of a video game to you?