Well, I'm an app developer myself and worked with many startups (as developer for hire) and also run my own app agency, so this is what I believe:-
1. A startup developer should be aware of the real implication of his code and also the target user group; he must know the sensitivity of rolling quality builds quickly; and its direct impact on business.
2. He should be an "engineering" mind and not just "coder" who can't think beyond their technology stack. He should always be concerned about the next generation of technologies and be ready to take the challenge for staying on edge of competition.
3. A little entrepreneurial attitude (at least ownership, accountability, time management) is always important.
And that's it, rest can be managed or fine tuned if attitude and approach is right.
I've personally experimented with my developers and figured that if you just change the "approach" with which they take the work and give them little more sense of "ownership" (combined with offering tangible incentives of success) their whole efficiency goes to another level and also the outcome product becomes better.
Btw, anyone looking for a developer right now can contact me and I can get them in touch with real vetted developers (and not freelancing site kind of guys) on my responsibility and professional image.