This question has always has been difficult to answer but listed below is an interesting article by Allan Collins from 'Success in HR.'
Alan, a proven HR Executive, takes the opportunity to share 50 things that he has learned from twenty-five years in the HR trenches based on his rich experience.
Moreover, it is an interesting article that encourages HR practioners to look beyond their functional boundaries and add value to the business http://www.successinhr.com/50-things
You are welcome to share your own insights or learnings.