I think this is one of the most difficult rules to abide by because a lot of very talented people say they would like to start a company, but never will. When it comes down to it, they'd rather be unhappy at their job and receive a steady paycheck than take a risk and start their own company. Do you lower your standards and start a company with someone who may be talented, but may jump ship at the first job offer elsewhere?
It takes time to find out what people are really made of. From my experience, people who say they want to start their own business or be their own boss really just want to be a mid-level manager. Other people have the "Entrepreneurial ADD" and will never be able to stick to one idea from beginning to end. Others get scared of the prospects of late-night coding and early mornings, and decide to go to grad school instead (how does that make any sense????).
What's the solution for finding good cofounders? I think at some point everyone comes across someone who would be a perfect business partner, but has different ideas. Start working to form a business with that person regardless. It's likely that the business will evolve into something that neither cofounder originally anticipated. So getting everyone on board with the same idea may not be all that important, you just need to be able to work together.