It's nearly all about the people if the startup is early stage.
- Do you believe in their ability to execute?
- Are they transparent and communicative (do they listen?)
- Do they have the appetite to deliver on their vision and have a sense of urgency about them to do this?
- Are the co-founders complementary in their skill sets?
- Is the CEO a leader? they don't have to be Fortune 500 material, just able (and willing!) to delegate, hire and inspire.

Yes you need to check the companies basic documents, due diligence, ensure the valuation is not crazy etc etc, but at the early stage it's really all about the people disproportionate to anything else.

If there is ONE other thing which isn't directly people, it's do they understand where this investment round takes them to (most often, another investment round) so are they raising enough money to give them enough runway to deliver on whatever is needed in order for them to raise another round of funding; in a seed or angel round very broadly that's usually 18 months of runway (as they'll have to start raising again after 12 months).