I just finished Mafia II and thoroughly enjoyed the experience. The graphics were good enough to simmer me in a great environment (late WWII / Early 1950's New York City) that I had only gotten to experience in movies before, the characters were interesting, the writing was good, the missions were engaging, and I found the end to be completely 100% satisfying and thematically appropriate.
AND I have zero interest in doing any of the DLC. None.
For me the story is over and while I enjoyed the world I feel like the time I spent in it was all the time I desired to spend in it and any DLC would probably lessen my memories of the game.
But what about you? If there is a game that you finished and liked playing - what is it that makes you NOT buy DLC in those instances?