I was watching a stream and one of the announcers offhandedly said "Meta Knight is banned" as if that is a foregone conclusion. I have never seen Meta Knight do much of anything, but then, I'm hardly a professional Brawl player.
What is it about Meta Knight that makes him so much more powerful than the other characters that the community resorted to banning him in competitive play?
From Smash Wiki:
He currently ranks first on the Smash Bros. tier list due to his extremely fast attack speed, his transcending priority found in a lot of his attacks (also found in lasers and needles), unpredictable recovery thanks to four recovery moves and Kirby's jumping prowess, ability to avoid being KO'd with his extremely fast up air momentum cancel, amazing edgeguarding ability, approach options, some fast and reliable killing moves, and amazing tournament results and match-ups. No match-up is thought to be a disadvantage for Meta Knight, although some matchups are approximately even.
His main problem is that he doesn't have any weaknesses.
The reason Meta Knight has been banned is basically because he's amazing in every single aspect of the game, and has no real weaknesses.
Meta Knight's recovery is hands down the best in the game: He has 5 jumps, a glide, and even after all that he can use one of his specials to recover. Add in his very good aerials, and a good MK player should never be gimped (that is, knocked away to where they cannot recover), ever.
On the flip side, Meta Knight is one of the best gimpers in the game, as due to his recovery he can go anywhere, and his down air has good range, power, and sends the opponent back at a downwards angle.
Although Meta Knight does not have a projectile, once he has a significant lead, he can be almost impossible to hit, as he can just run away from the opponent and use his high priority high range quick attacks to make it very difficult for them to hit him until the timer runs out. The most infamous form of this would probably be planking (grab a ledge, drop down, use two up airs, and grab it again), which is nigh impossible for some characters to get around (and until recently was thought to be nigh impossible for most characters), and in fact was a big reason in why tournaments added a ledge grab limit for matches, to prevent Meta Knight from timing out the clock in such a way.
Meta Knight's worst matchup in Brawl is another Meta Knight. The official matchup chart has only three other characters that go even with Meta Knight, and many characters he beats really really badly. This is further aggravated by the fact that in tournament play, half the opponents you'll face play MK. For example, there's only one really good Pikachu player (ESAM) at the moment, and Pikachu is pretty high tier. There's a ton of Meta Knight players (M2K, Ally, Anti, Sebrik, …), and it's rare for a national tournament NOT to be won by a Meta Knight.
So to summarize, Meta Knight is banned because he's broken to the point where he's used far more than any other character in tournament play, has no bad match ups and only a couple (arguably) even ones, as well as multiple broken aspects (his planking, as well as the infinite dimensional cape glitch) that require their own rules to prevent him from being even more broken.
Meta Knight is basically way overpowered in every aspect, he is a medium weight, high speed class with incredible recovery abilities. Even if you get a good hit on him he will need at least 300% damage to KO.
I took down all 3 of my friends with him in a free-for-all and they are all hardcore brawlers who are good with even the bad characters.
To sum it up stick two equally skilled players against each other and have one of them be MK and the other play as every other character in the game. Meta Knight will win every time.
bmoves can be used to recover.