For me it's the battery. Having to charge a watch every 24-48 hours is a complete non-starter, regardless of its functionality. I don't want to carry more cables or charging stations with me wherever I go. It's the single-most point of stress, especially when I travel.

If the battery lasted 7-10 days, it would be perfect. That's about how long my business trips take, so I could charge it to full beforehand and not worry about it at all during the trip.