Genesis, the major tournament in San Francisco.
Unlike the rest of the cast, due to his numerous jumps, glides, priority-ball specials, quick low-lag aerials, best airdodge, and powerful defensive moveset, MK has a "safe" option in almost any situation that results in the neither-hit result. As MK cannot be trapped in such a manner, MK is capable of continuously choosing safe options until punishment opportunities arise. This is why so-called "gay" Metaknight playstyles are so effective. MK is capable of abusing his safe options, and in some matchups, to an absurd degree- for example, the DEHF vs Dojo matchup, in which Dojo ran the timer out by simply remaining in the air using his aerials defensively the entire match (a match which a Genesis judge wanted to DQ him for, despite the fact that no rules were violated).
Due to these points, we believe that Metaknight is a character that is not in line with the rest of the cast, and thus not fit for competitive play.
Unfortunately, Metaknight breaks both the stage aspect and the character aspect. The only stages Metaknight has shown to do poorly on have been Shadow Moses, Bridge of Eldin, and other ridiculously one-sided stages. As for characters, Metaknight has shown to have only even matchups at best and has continually outshined every other character in the game. Because of this, Metaknight inherently breaks the counterpick system.
3. Metaknight has a perfect recovery and an abnormally safe ledge game.
One of the basic game mechanics that Metaknight bypasses is that of recovery. Metaknight's recovery is virtually perfect; he always has a safe option to recover, unless he has needlessly wasted them. With the combination of his multiple jumps, shuttle loop, two glides each capable of crossing clear to the other side of the stage, tornado, Drill Rush, invulnerable Dimensional Cape (as well as Infinite and Extended Dimensional Cape to cross the stage), and quick aerials with little ending lag make his recovery not just safe, but unstoppable barring a mistake on the part of the Metaknight. By bypassing the entire edgeguarding aspect of the game, Metaknight takes less damage and virtually never gets gimped, which, when combined with his excellent momentum cancels, greatly minimize the seeming "lightweight" disadvantage.
The pro-ban side has prepared a video demonstrating why MK's recovery breaks normal standards of recovery and breaks the game's normal edgeguarding mechanics.
http://www.youtube.com/watch?v=YDCG6NF10oU
Metaknight's extremely powerful recovery ties in to the issue of safe options, which will be discussed shortly.
Another point is Metaknight's ledge game. Typically, in smash, being on the ledge is considered a disadvantaged position. In Brawl, there exists a subset of characters who are safe on the ledge as they remain on the ledge (playing on ledge the entire match is referred to as "planking"). These characters include Mr. Game & Watch, Marth, and of course, Metaknight. A ledge grab rule was created to discourage planking from dominating matches, and for the most part, it has been effective- except where Metaknight is involved. Metaknight is the only character in the cast capable of stalling offstage in an unapproachable position over an extended period of time in between the ledgegrabs (and further, gliding under the stage to the opposite ledge if threatened). As a result, we see Metaknight players continuing to abuse such strategies in spite of rulesets designed specifically to stop it; and said rulesets were instituted for the most part due to Metaknight users abusing planking in the first place.
6.
Metaknight detracts from the metagame.
We have rules in place to prevent planking and stalling, which have become big issues. We’ve tried various amounts of ledgegrabs being allowed, but planking and stalling have remained issues. Meta Knight players have found ways around these rules, and have stalled. See Dapuffster v Plank at Apex, or, more recently, Dojo v DEHF at Genesis for examples of stalling that cannot be limited by any reasonable rule. Air camping or air planking, whatever you prefer to call it, makes many matchups unwinnable, and not just against low tier characters. Diddy Kong, Falco, Ice Climbers, Marth, Donkey Kong, Lucario, and to a lesser extent Wario and Snake, have their matchups tilted strongly towards Meta Knight’s favor because of air camping, and are all in B tier or higher. These techniques that skirt the lines of the rules but cannot be truly banned or limited make Meta Knight a broken character worthy of a ban.