At the risk of being overly literal here, I have always found it odd that some people wish their opponent 'good luck' in any game -- but especially in Dominion.
While I suspect it's usually shorthand for "Hello, let's enjoy this game together!", if it's taken at face value it makes very little sense. In a competitive game you hope to win, why would you wish that your opponent is lucky? Are you wishing for the absence of really bad luck?
A more cynical interpretation is that it's a form of taunting or gamesmanship, as in "Good luck, you're going to need it!" In virtually any other competition I can think of, if one opponent said "good luck!" to the other before the match, it would most likely be interpreted this way.
Does anyone else find this odd?