You've left out the quality of the management team and particularly the experience and temperament of the CEO. While working for an Angel investment company and VC Fund, I learned that a brilliant founder with a huge ego and a failing marriage can be the death knell for a start-up.

Regardless of the brilliance of the product/technology/business plan, if the person at the top making the big decisions is an emotionally immature person or has family troubles, he'll do stupid things that can derail the business.

Smart investors always look for who is going to run the company and what their track record is.