Large markets, either existing or newly enabled by the innovation at hand.
Will you disrupt a large existing market? We love breaking monopolies. Or can you find a billion dollar market (versus a million dollar market) with a credible path to grow?
Short innovation cycles
Startups by nature take on more risk than larger, established competitors. However, the nature of startups also adds significant difficulty to getting large, capital-intensive projects up and running. Instead, we look for rapid innovation cycles so proof points and results are available quickly (whether good or bad). That said, we also break our own rules; we have invested in a nuclear reactor with long innovation cycles because other factors were compelling.