Unless I was way out of it, I thought they banned Feral Chaos because he was busted as all ****. Like, he 6-4'd the best and 9-1'd the worst. Granted, I never found people to play who were competitive (I'd beat up everyone I knew while playing Jecht at a level handicap of about 15). Also ExDeath was busted yes, but he had a ridiculously high tech skill barrier [perfect Omni-blocks? that timing is so annoying...] - Cloud spamming Meteorain was still a **** to deal with because high guard didn't block it and if I remember right the invulnerability frames of Omni-block weren't always big enough to avoid getting nailed...
That all being completely irrelevant to the topic.
Meta-Knight as a character in Brawl may have been overpowered, but he's not necessarily broken. Meta-Knights who use the Infinite Dimensional Cape are probably broken (invisible approach from across the stage? intangibility on most of the approach? That's pretty broken). And Brawl Meta-Knight in P:M would be broken - those up-air strings would be so stupid on so many levels, not to mention Mach Tornado for 20+% (also his aerials, with their Brawl lag, can have reduced lag, thanks to L-cancel? Yeah they had to nerf him...).
To answer your question, it's probably a combination of utility and breakability - Falco's lasers in Brawl are very useful, but powershielding, good jumping, and keeping close can render it moot. Fox's utilt has high utility for combos and low startup/cooldown but it can be beat out by a variety of moves. Mach Tornado is considered broken by many because of fairly low startup, fairly low ending lag, high utility of the move (can extend combos [off the top KOs?], chase people, run away while protected, approach), and it's hard to deal with - not many moves easily break through it [at least that are quick enough to deal with it at close range] and it's often used when ground attacks and short-hopped aerials aren't an option. This plus its huge degree of safety compared to most moves (you can run away with it, or approach, if you land the final hit of the attack you can fastfall to safety and will almost never be punished) means that it is considered broken by many.
That said, this is all probably a subset of Umbreon's earlier post about breaking the standard rules of gameplay.