The previous comments are valid, but there is also something to be said about the tangent industry you'll be working in. Obviously, you are into web 2.0 and the culture but it really helps to be interested in more than that.
For instance, if you are deciding between last.fm (music) and pumpone.com (fitness), all else being equal you should choose the industry you're more interested in. Being able to relate to the users is ALWAYS beneficial.
What comes to mind initially:
The start up has to have a clear intention:
-demonstrates clarity in the unique tasks that need to happen
-demonstrates trust in that others are creatively capable in doing specific tasks independently
-demonstrates the simplest process for arriving to first prototype to be tested by users
Couple ideas came to mind. I'm sure there are others.
1. Would you be working on a product with a potentially huge market? Or is it simply a "cool" product?
2. How do you like the founders and other people? Ask them: what are the 3 best things they like about the work, and the 3 things they detest the most? What's their history?
For instance, if you are deciding between last.fm (music) and pumpone.com (fitness), all else being equal you should choose the industry you're more interested in. Being able to relate to the users is ALWAYS beneficial.