As much as I love the Tube map, and as much as I'd hold it up as a good example of design, I often worry about how easy it is for tourists to use. That's the kind of thing I worry about.
I saw this on the Metro in Barcelona.
It's brilliant, isn't it? You know exactly where you are and exactly where you're going, no matter what language you speak. Nice and simple, nice and easy.
That's great usability.