I wrote two articles, one on what you need to know about the startup before you join and the other on how the discipline of business thinking matters more than the idea. Here is a simpler advice that I received in a conversation with Professor Rashi Glazer of UC Berkeley, on taking any job i general:
First ask yourself, do I want to want to work for the company that wants me? Why do they want me? What value can I add to the company? If you cannot figure out your value added, ask them how you can add value. If there is no value added, why take that job?The other advice I received is from Lynn Upshaw, another marketing processor at Haas School and a Brand Consultant.
The other thing to keep in mind is, you don't work for the company, you work for your boss. How do you think you will make her/him look when they hire you? If you cannot make your boss look good, what is the point?
There are things you need to care about joining a startup for your first job or when switching careers. In a small shop you do get to do a variety of things. But you have to ask who is around me who has been down this path and can teach me. If you have no one to learn from within the organization and you are learning from outside as you execute, you are not learning much. For all the bad rap on large organizations, they have lots of people who have been this path a few times. If you reach out to the right people you can learn a lot faster.