I can't stop pointing out additional cluttered design features and explaining more clean games. I think it's important to see a range of different clean games that vary in depth, complexity, skill, genre, style, theme, etc. Plus, doing more examples helps me calibrate my scale.
Input buffering is when the input signal is sustained or carried over for longer than it is inputted into the "controller" by the player. This feature is the intersection of input, player/character states, and mechanics design. Traditionally, games are designed with no buffering. When you hit a button to execute an action, if the game state isn't ready for that action to happen, you get nothing. I call this the dead state for that input or action. Typically, games do not provide any feedback for dead state inputs. You would have to release the button and press it again for another attempt at the action. This kind of design is great for creating a very clean relationship between intent (inputting into the controller) and action. It also works well for digital inputs that are either on or off like buttons or the individual directions of a D-pad.
In the same way that hitboxes and cancels can be tuned to make a game play and feel more like its fiction, buffer frames can actually be used to make a game feel more precise. In a simple platformer with no buffer frames on the JUMP mechanic, you can only JUMP when your character is standing on the ground. So if you're trying to time a JUMP just as you land from the air, if you hit the button in the frame just before the character hits the ground, you'll get nothing (dead state). A buffered JUMP can make this precise timing challenge a little easier so that players that are slightly off still get the result they want.
Precise timing challenges and tight windows can benefit greatly from a buffering system. Take the classic Pac-Man game. When you move through the hallways, the motion is smooth and continuous. When you want to dart down an adjacent path, you'll find that as you approach the turn if you start holding the new direction early, Pac-Man will make a very crisp precise turn.
Super Smash Brothers Brawl has an input frame buffer of 8 frames. That's 8/60th of a second (.133 s), which is faster that your fastest reaction times. Don't believe me? Take the test for yourself. We, B.E.S, designed the game REFLEX so you can measure your own skills. As long as the buffer for inputs is faster than your fastest reflex speeds, it becomes difficult for a player to notice the slight disconnect between inputting and seeing a buffered action.
Buffer frames can take the edge off of playing by widening precise timing windows. One downside is with greater input buffering the player is more likely to feel a disconnect between his/her inputs and actions. Guitar Hero is a very popular music-rhythm game. One notable difference between it and Japanese music games is the timing window for hitting notes. Japanese games love to be very precise and reward players for being exactly on beat within a few frames. Guitar Hero has much more leeway and counts a hit as a hit no matter how well you fall on the beat. The result? Losing players spam more in Guitar Hero hoping to catch a few notes. Also in long sections of repeated notes, it's easy to lose the beat and slowly trail off so that your inputs are steady, but they're just steadily off of the real beat. During these momements it feels like I'm not even playing the game/music.
Another downside to input buffering is sometimes the game will carry on with actions or convert your inputs into moves you never intended simply based on the way the game/character states are programmed. For this reason, there can be a glaring disconnect between the intended action and resulting action for buffered mechanics. For these reasons even well designed bufferable mechanics can easily clutter a game. The more complex the game with buffered moves, the more likely there will be buffering issues. Noticable buffered mechanics detract from the cleanness of mechanics under the "direct" category.
Take BOXLIFE for the Nintendo DSi. (watch the trailer here)
.9*(1*.959) - 0 = .8631
Multitask is a perfect example of a game with only a cleanness issue of simultaneity.
1*(1*.833) - 0 = .833
If you think my cleaness expression arbitarily reflects my on preferences and biases as a gamer, I encourage you to play all the games for yourself and compare your experiences. With the math, I can more easily see that BOXLIFE is cleaner than Multitask, which is not an issue of style, graphics, or genre.