I was thinking about the importance of a good theme, and i think essentially it is like a free IP. The theme is what draws people to your game, and allows you to get a cross over to people who would otherwise not be interested.
Many people would play a Starwars themed game just because it is Starwars. Wingspan struck a chord with the birding community who are weirdly in love with birds, and became super fans.
I like to stay in the introductory/gateway game area for my designs. Smaller, cheaper, and easier to test etc. it seems to be important to get something that appeals to a wider audience. people who would be otherwise turned off by "nerdy stuff" may enjoy a game about growing plants. Some people at the dice tower will only truly enjoy a game if it is pirate themed :)
What are your favorite themes? What seem to be the trends? What are unusual themes you have heard of?