What are some qualities which make a good start-up engineer?

Be well connected to start up groups.

Follow all tech VC and tech guns for latest trends.

Be adaptable, love your idea but be ready to improve it.

Constant re positioning of the product.

ROI - Understand this term and but don't start living with it.

Very important: knowing when to pay off technical debt. It can be more of an art than a science, and at times it's blatantly obvious; I'm not sure whether this is an acquired or a native skill, but it's important.

The ability to learn very quickly: I believe this implies some (high) level of intellectual curiosity, as the best learners (in my experience) are also very intellectually curious. This doesn't just mean learning new-to-them engineering knowledge, but also new-to-them people/management/social/etc. knowledge.

Honesty, personal responsibility. I've seen otherwise honest engineers show their compulsive-liar side when put under extreme pressure (usually after very little sleep), or called-to-task for some error of judgement. Stand up, admit mistakes as usual. Learn from the error.