There are quite a few but if you forced me to pick just one then I'd also go for battery life, a minimum of 5 days and ideally 7+ days between charges.
A close runner up is I think a lot more attainable for Apple in the near term and that's an Ultra-like action button on the non-Ultra models plus software support to configure single, double and long presses on it to launch my most used apps. An action button getting announced as a surprise feature of series 9 that the rumour mill somehow missed is probably the only thing, assuming that Apple doesn't nail battery life this year, that is likely to get me to upgrade my series 7. I am so underwhelmed about what the rumours are saying about series 9.
A close runner up is I think a lot more attainable for Apple in the near term and that's an Ultra-like action button on the non-Ultra models plus software support to configure single, double and long presses on it to launch my most used apps. An action button getting announced as a surprise feature of series 9 that the rumour mill somehow missed is probably the only thing, assuming that Apple doesn't nail battery life this year, that is likely to get me to upgrade my series 7. I am so underwhelmed about what the rumours are saying about series 9.