A setting to choose Fahrenheit or Celsius as the default temperature would be great. Probably within the Profile > Personal Info settings.
Currently, it's possible to use C, but its usage is inconsistent across devices.
For example, right now:
Nest Thing shows temp and controls in F even if Nest is set to use C
SmartThings SmartSense Multi doesn't show any indication whether F or C is being used, however it works in C for me (presumably because that's what I've set within SmartThings). The parameter the sensor passes to the next Thing is named "Temperature (Fahrenheit)" even though the value is in C.
While on the topic of Metric/Imperial - having the option of km instead of miles in the Location Thing would be great, too!