For me, attitude and commitment are the most critical aspects when hiring a person. Everything else can be taught.
Of course, it depends on what you are hiring for. When hiring a domain expert, you want to make sure that the attitude is right, the commitment is 100% and that the person does have the expertise in the domain that you want to hire him/her for.
Another thing that I have learnt, sometimes the hard way, is that it is important to ensure that the person you hire is also driven with the excitement of the startup. If a person is not super excited with the concept or opportunity or the startup, no matter how committed the person, and how right the attitude is, he/she may not be able to or be willing to survive the challenging times that most startups invariable go through. Some of the challenges, especially due to lack of resources & capital, make the journey incredibly tough. And only those really driven can go through it with determination.
It is important to clearly communicate to potential hires about the challenges and realities of working in startups. Explain to them the limited resources that they will need to work within, clarify that processes are yet to be defined, explain that most things will be ‘figured out as we go along’, etc. etc.
Ensure that they clearly understand the work environment and work culture before they accept your offer to join.