The other important thing (that I forgot to mention) is that the same thing is true in reverse. There are some cards that are really good in competitive commander, but are not so good in casual.
Take for example,
Ruric Thar, the Unbowed. If this guy is your commander, he stops storm decks, makes stax decks
think twice about certain stax pieces, he kills most infinite loops, and makes other combo decks think hard about whether they can survive their combo turn or not. You also have to remember that non-creature spells tend to be more powerful than creature spells, which causes damn near every other deck to think hard about what they are casting.
But in casual commander, where creatures are more common, Ruric Thar is a mild irritance. Different cards work well in different metas, but some cards (like my nemesis
Bojuka Bog) just have better versions out there, regardless of meta.