what are some of the key elements that might be contained in a corporate's value proposition?

Answer 1

Usually a value statement defines the core values that governs the companies method of operation in an ethical manner..
IE:
Open and honest communication
Treating employees with dignity and respect
Providing value to the customer/employees/investors
Ensuring compliance to health and safety and environment
To be a good corporate citizen
These are a few

These are not to be confided with the mission statement which is usually different.