If you got into this game before starting or finishing the books, theres a pretty good chance you had some plot points spoiled for you.
One of the worst ones for me was Make an Example:
I was 3 or 4 chapters away from Eddard's beheading when i was looking intently at this card. When i realized who's head it was i was so pissed, i decided to not play the game until i finished the books. That didnt last long though, the games too good and i'm too slow at reading instead, i just tried not to look too hard at the pictures or the flavor text while i played. Anyone else have stuff ruined by cards?