Why is William Shakespeare important?
His stories (most of which were not original to him) and the words he had his characters say (which were very original and characteristic) are so well known that they are constantly alluded to. Ask anyone to name a play, and odds are they will name one of Shakespeare's. People constantly use phrases he created and words he coined.
He has a special place in the history of drama as well. His characterization and use of dramatic subtext raised the bar considerably for the production of serious dramatic works. It is a bar which few have reached.