Based on 20+ years recruiting technical and executive talent, mostly for startups, the biggest reason people join a startup as opposed to a Facebook, Google, or Oracle is a function of their tolerance for risk and a deeply subjective understanding of what they desire in their job. That desire usually boils down to the real culture, and culture boils down to values.
All startups are not the same (any more than all large companies are the same); each reflects the values and attitudes of its founders.
Some people prefer startups is to be on the "bleeding edge" of whatever technology is involved.
Equity is important and stock grants should reflect the company's publicly stated values and culture.
In short, people join startups because they "fit" and the environment makes them happy. What it does not make them is better than the person who works at a large company (Facebook is a large company).
When you are hiring look for people who understand your vision and share your passion and create a culture that will attract them.