Wow this has gone completely off topic hasn't it.
Anyways, with all the talk about Ambassador, might as well talk a bit about the design of it. It is absurdly powerful, is very game-warping, and yet it exists. I suspect that it's for similar reasons as to why Chapel exists. Ambassador is a card that plays differently as a player's experience with the game increases. It is only game-warping when you realize it is. Personally, I didn't realize how good Ambassador was until Obi Wan Benogi opened Amb/Amb against my Chapel/Silver and crushed me. It doesn't say "trash" anywhere on the card, does it? Just some mentions about "returning to the supply"...
So, it appears that from Chapel and Ambassador, we can derive a general rule.
"If a card has to be overpowered, it should be overpowered in a subtle way"
A major component of Dominion is the puzzle aspect, where the players try to figure out interesting card interactions and how to best exploit them. So if cards you design all have a straightforward purpose, that's not good. Multipurpose cards => more interesting cards. Don't let complexity creep sneak up on you though.
And just for fun, here's the opposite of that statement, which has some element of truth.
"If a card is underpowered, make the card seem overpowered."