So far, the posts in this thread have mostly focused on aspects of preventing electrocution, although lightning protection was mentioned maybe once.
There are other issues such as fire prevention, EMI (electromagnetic interference) suppression, and also legal issues. I have done some looking into these issues with regard to inverters because I have had interest both in solar panels and also battery backup systems.
For example, there are some solar panel systems for which the NEC (National Electrical Code) requires that one of the DC wires be grounded.
In order to discuss these issues better, I have created a PDF document that includes text as well as some drawings to help explain some of the issues. It's about 3 pages long. I think most of what of I put in there is correct, but if some of you think there are errors, I would like to know about them.