Ok people, we have to keep a few things in mind here.
For all the people who say "he is in the game, deal with it". Akuma was in the game, so deal with it? The point is we are trying to define MK as broken.
Now to explain why MK is broken.
Most fighting games have some sort of health meter you need to take down to kill the opponent. In brawl, you can die at 20% or 200%. The ability to recover is unique to brawl and a characteristic of a each character. Lucario doesn't do damage when he recovers, olimar's is easily edguarded, Wario without a bike is useless, Snake can be grabbed out of his. All people have some sort of flaw to them. Not MK, he has 5 jumps, 2 glides, and 3 other recoveries. He won't be edguarded, ever.
Also, characters have weights. To balance out those weights, typically lighter weights are faster and weaker. Heavy weights are slower but more powerful. If a character has a good ground game, then his air game will lack. If a character is fast and strong, then their recovery will lack. If a character has a poor oos game, then he will be very offensive. Every character has some sort of balance to them.
Once again MK is an execption to this rule. His ground game is awesome. Dsmash is fast and kills, Fsmash is unpunishable, his ftilt has so much range, and is quick, his dtilt is the ultimate ground spacing tool because it is fast, trips, and can lock oppoents. His grabs are strong and lead well into his moves. All of his aerials have top priority. His fair hits 3 times, with a good amount of range, and knockback. His nair comes out on frame 3, is lagless, lasts the majority of most characters dodges, does 22% and has an incredibly high amount of knockback. His dair is the ultimate gimping tool. It also comes out before most people can react, can be lagless. His up B kills at extremely low percents once off the stage. And he has 5 jumps. But he is light so he is balanced. Not really. He has the best momentum cancelling in the game due to his uair which happens to be lighting fast. It is so fast that there is no point in airdodging because the next one is already hitting you. Therefore he no longer has that weight to balance him and dies just as late as the rest of the cast.
Also he is a shorter character, so its not like he will get hit with many of the moves that larger characters get hit with.
"But other characters **** too, Snake will be the next MK"
In the sense that he is the best, yes. In the sense that he is broken, no. Snake is flawed. He is able to be chaingrabbed by D3. When he is falling in the air, his options are very limited. If he gets edguarded, he is forced to do about 70% to himself just so that he can live. How often do you hear about MK being countered? Never. What about snake? A matchup is not the actual outcome, it's merely a prediction of what should happen if both people play their best. I have seen countless times talk of snake counters ie. D3, Olimar, Rob, Fox. Even if they aren't necesarily the worst, people believe that characters have advantages over snake.
The other important thing about MK is his learning curve. Most other S tier characters require much time and practice to be played at medium to high level of play. 1 year IC's=1 month diddy=1 week D3=1 day MK. He is the ultimate secondary character. You already have a main that you work hard to get better, but you have a few bad matchups, so you pick MK. It would be different if he required months to become that good with him, because not many people would work that hard, and only a few people would be able to use his moves to their full potential(see snake). Thats not the case, he is easy to use, and the best.
Each of those things by them self doesn't make someone broken, but all wrapped together in a tiny little ball of priority makes MK broken.