Helping users form an effective mental model of the system: what different devices do, and how they are interconnected. When is it appropriate to explain the system model – how things actually work – and when to simplify so they don’t need to concern themselves with technical details?
Effective composition: distributing functionality between devices, to suit the capabilities of the devices and context of use.
Appropriate consistency: how to determine which elements of the design should (and should not) be consistent across different interfaces, considering e.g. terminology, platform conventions, aesthetic styling and interaction architecture.
Continuity: how patterns of connectivity unique to IoT can cause discontinuities in the UX between devices, and how to handle these in the design.