Six mana is pretty steep, but in a green deck, thats not that hard to achieve
That prevalence will warrant a double-ban in April, by all historical standards, and you don't want to hold the bag when that hammer comes down
For two mana you can get a one-turn Panharmonicon-type effect for everything you play after it
When gold cards were first introduced, not only were many of them not particularly strong, but they had a unique restriction all their own, because these cards weren't just special
Kefka is a little difficult to enable at three colors, but if you get it early enough you can build towards it
Even in Commander, the card can put in some serious damage unless your opponent is willing to trade three mana to remove it