Soren Johnson, the lead designer of Civilization IV, has a great duo of posts on the difference between theme and meaning. His point applies to any type of game, including boardgames, which he cites liberally (Ticket To Ride, Diplomacy, and others) throughout the piece. Click here for Part 1 and Part 2.
The executive summary: game designers should bring theme and meaning into close alignment. It's the best argument against "pasted on themes" I've seen.