7. They have to be able to support a full day's use on a single charge, and charge conveniently.
Digital watches could go for months, sometimes years on end with a single battery. Today's cell phones barely make it through the day without keeling over. A smartwatch can't afford to have as bad a track record with power consumption, lest people feel they now have two things that can die on them instead of just one.
And as far as charging goes, here's yet another no-brainer use case for wireless charging. Instead of fumbling with a watch and a really tiny connection port covered by some flimsy rubber plug, why not just take the watch off, dump it on a charging mat next to one's desk or bed, and forget about it?