Theme definitely plays into what most people think of a game, and even the best mechanics can't get past that. I had to read a paper for class this week talking about this, and they mentioned a game called "Genocide Tetris." It had the exact same mechanics as regular Tetris (an absolute classic), but instead of the abstract blocks, you were throwing Jews into a gas chamber. Now, obviously this is going to disgust just about everyone out there, but the game plays exactly the same. If things were only about mechanics, then it wouldn't make a difference.