It must be useful, and this usefulness must be measurable.
The less interaction required, the better. For example, why use both hands when you can use just one.
Device should not disturb you from daily tasks.
It must be safe to wear.
Device must be easily extensible.
On software level:
It should be relatively easy to write your own extensions.
The only thing required to change the client code is a text editor. By using interpreted languages we can make it possible to edit the code as soon as you get some idea.
On hardware level:
Ring, SPI and !I²C ports to allow pluggable modules.
Excessive hardware features. Things like barometer or accelerometer are not required for basic operation, but they provide freedom of thought for developers.
All development files must be released under a free software license. Besides boosting development speed by letting others to contribute, these files might come handy for other projects.