Having worked at startups for a very long time, leadership is the most important area for me. Not only are these the people that can truly drive the idea forward but are these also the people who can help you succeed. I've worked for startup leaders who wanted experience but didn't know what to do with that experience.
For me it’s first and foremost the work that they are doing. If I’m passionate about the work then I can fight through most odds. And then come the people and culture. Although they would have to balance each other out to some extent. I have previously left great work because of the people and culture because the imbalance was too great for me to balance.