What are the differences between various versions of Citadels
Took some time to compare the rules, but man there's more than I thought. Some are more subtle, others are complete game changers. Here's a complete list (unless I missed any):


Poor House


US: costs 5 gold to build.
EU: costs 0 gold to build.

Removing characters


US: First put down the face-down card, then the face-up cards. If needed, replace the king from the face-up cards. The starting player can not look at the face-down card.
EU: First put down the face-up card, then replace the king if necessary, then play the face-down card. The starting player can look at the face-down card.

End of turn


US: After all the characters have been called, each player returns his character card to the Character Deck.
EU: At the end of your turn, place your character card at the center of the table.

Discarded cards


US: Face-down at the bottom of the deck
EU: Face-up discard pile

Character abilities


US: Used after performing an action (choosing gold or cards)
EU: Can be used before or after performing an action

Queen ability


US: Receives three gold if you are sitting next to the King or the Emperor (eg the person with the King/Emperor card).
EU: Receives three gold if she sits next to the player with the wooden king piece.

Warlord ability (in combination with Bell Tower)


US: May not destroy a district in a city that is already completed by having eight districts.
EU: May not destroy districts from cities that already has eight districts.
receive any gold from the Poor House or from their Alchemist ability.

Emperor ability


US: The player who receives the Crown chooses to give you either one gold or one district card from his hand.
EU: (no such rule exists)

Diplomat ability


US: Used at end of turn. Can not exchange with a completed city.
EU: Used at any time during your turn. Can exchange with a completed city.

Identical districts


US: You may not build a district such that you have two identical districts in your city.
EU: (no such rule exists)

Bell Tower


US: Bonuses are awarded for seven districts instead of eight.
EU: (no such rule exists) bonuses are awarded for eight districts.

Cemetery


US: If you are using the Diplomat you must remove the Cemetery.
EU: If you are using the Diplomat, the Cemetery loses its function.

End of game scoring (in case of tie)


US: Most points wins. In case of a tie, only look at points from district cards. If there's still a tie, whoever has the most gold wins.
EU: Most points wins. In case of a tie, only look at points from district cards.



Everything on the list is based on these documents


English
German
Dutch
French


I haven't found any differences between the German, Dutch and French versions, they seem to be mutually consistent.