Consider What Constitutes Important Feedback
Technically, the developers determine the amount of information a game has to present. What I mean by this is outside of hacking a game and analyzing the code the developers set the maximum amount of information the game can present. If a developer doesn't want the player to see everything in the game world at all times to make the gameplay more interesting (like the limited player perspective in an FPS or fog of war in StarCraft 2), then we can't simply point at this lack of potential feedback and automatically fault the game for clutter. We can always ask for more feedback or more information from any game. So figuring out when a missing piece of feedback clutters a game and when it doesn't takes a bit of careful thought.
Issues of important feedback are measured according to player mechanics. In Mario's case, players need enough time to JUMP over enemies or slow down to a stop. Fortunately, the camera scrolls in such a way to keep Mario at about the middle of the screen. This means the vast majority of interactions will be near the middle of the screen and clearly displayed. Even at running speed upcoming enemies and structures appear with plenty of time for players to react based on average human reaction times. This is typically what I mean when I say that SMB is designed so that players can make informed decisions. Does making informed decisions equate to having adequate feedback? Should informed decisions in real-time games always be determined by reflexes? I say no and no. After all, reflex, sensory, or information overload can make measuring the amount of time needed to "properly" react to a presented stimulus difficult to gauge.
Another idea to consider from the Mario example is the fundamental idea of looking for adequate feedback in terms of playing a game successfully to completely avoid danger. But there are many, often emergent, ways to get trapped or lose an advantage in SMB. Just because you're in a losing situation doesn't mean the feedback or cleanness of the gameplay falters. Is losing an advantage or "challenge fairness" really a factor of a game's cleanness? Again I say no. A punishing platformer may be cruel or cheap, but that doesn't mean it has cluttered mechanics, feedback, or camera design. Or a riddle-puzzle game may require incredibly obtuse logic, but this doesn't mean it doesn't provide clear feedback for your many failures.
Informed decisions or adequate feedback specifically looks at the action-reaction relationship of player actions. How elements move from your attacks, where contact is made between two hitboxes, how much damage is done, etc. So here's how we determine feedback clutter; after executing any player action if it is possible to miss the visual, aditory, and/or tactile feedback then the action is cluttered. Remember, the potential for clutter is still clutter.
It's great when gameplay interactions have multiple sensory feedback design. Sight, sound, and rumble feedback provide more ways to inform the player in the event that the bandwidth is is overloaded or cluttered for any one of these channels. Camera design typically only affects a game's visual feedback. Because roughly 80+% of the total feedback bandwidth is generally communicated via visual feedback, we can generally assume that lacking visual feedback is what creates the real clutter. Comparatively, lacking sound isn't as big of a deal and even less so for missing rumble.
Putting It All Together
So if we take the inherent perspective limitation of an FPS as marking the highest level that the player can be informed (meaning even if you can't see everything at once, this limitation isn't clutter), we can simply look at how various gameplay interactions play out to determine if adequate feedback is given and/or obscured. Now we know that being disadvantaged by enemies that shoot you off screen isn't necessarily an issue of clutter. Nor is having elements spawn, move, or fight out of view.
Still sticking with first-person shooters, in terms of typical player actions bullets tend to be fairly clean. When you shoot a bullet, it travels so fast that it's difficult to turn the view away to miss it hit some target. So you're bound to perceive the complete action and reaction of shooting. However, the greater the distance and the slower the ballistic the more potential clutter there is. Shoot a bullet far enough and you won't be able to hear or see it hit anything. Grenades tend to be cluttered elements. Because of the timed explosion and the ability to lob grenades over 3D geometry, it's easy to miss the reaction of your grenades. Sure, you maybe able to hear where the explosion is especially with precision positional audio. But it would be harder for you to determine if targets are damaged and how they're influenced by the grenade force with just audio. Again, the potential to miss feedback is all we need to consider here.
Consider Stylized Feedback Restrictions
What's a Blooper doing here out of the water!
What about games that obscure the player view or limit other feedback elements for effect? Like the Blooper ink on the screen in Mario Kart DS/Wii (see image above). Or the bloodied-blurry vision after taking damage in the Call of Duty series. Or the ringing silence that overtakes the soundscape after standing too close to a grenade explosion in Halo: Reach. Or the temporary vision blindness after taking a hit in Wii Sports Boxing. Yes, all of these effects are cool. But what makes them cool is that they're cluttered. For a brief moment your senses are stylistically blocked and you have to rely on less information than you started with. The cluttered effects in many of these examples are part of the challenge/gameplay design. But as I explained previously, the difficulty, challenge, or the fairness of a challenge is not the same as its cleanness. Stylistic or not, clutter is clutter.
It has been unexpectedly difficult to define and refine my criteria for game design cleanness. Simple 2D games are easy enough. However, tweaking the criteria so that the same standards apply to all types of games was tricky. 3D games can inherently create a lot of clutter just because a 3D world is presented on a 2D screen. Furthermore perspective alters the visual presentation of all static objects. Just viewing an object up close, from afar, or at different angles affects what you can see of it and around it. While 2D Mario is always the same size relative to game world, an object in a 3D game can lose a lot of detail while possibily obscuring other feedback elements just by being a certain distance away from the player view.
I wanted to get a clean# for Halo: Reach. Maybe I'll get to it and Super Smash Brothers Brawl in the future.