I like the temperature converter one because it immediately introduces some of the more fuzzy design questions and annoying details that pop up when you implement data binding.
Such as: when exactly should the other side update? When the user switches focus (where to?) or on every keystroke? Does it make sense to a user that they temporarily see the Fahrenheit value for 2°C if they really want to type in 20°C? What happens if they type an invalid value (or clear the box, which they often have to do)? What happens if they type an incorrect value and then type something into the other field?
Etc etc.
I think there are a lot of interesting tensions between a "formally correct" system and one that feels usable and intuitive to use. Especially with bidirectional data flow, they show up quickly.
Such as: when exactly should the other side update? When the user switches focus (where to?) or on every keystroke? Does it make sense to a user that they temporarily see the Fahrenheit value for 2°C if they really want to type in 20°C? What happens if they type an invalid value (or clear the box, which they often have to do)? What happens if they type an incorrect value and then type something into the other field? Etc etc.
I think there are a lot of interesting tensions between a "formally correct" system and one that feels usable and intuitive to use. Especially with bidirectional data flow, they show up quickly.