The degree to which he was able to mobilize the kingdom for war was unprecedented, and I think the most significant aspect of his reign. At one point in I think the War of Spanish Succession, France had more than 300,000 men in its army, an unthinkable number in the early eighteenth century. His administrative abilities, while probably based on an unsustainable social structure, were also considerable, and overshadowed by much of the "Sun King" propaganda.
What impressed me most about him was the way that he was able to be so forceful in creating a relatively absolute monarchy. It must have taken a lot of personal charisma and ability to make people be willing to go along with that. If it were easy, it would have been done before. That doesn't necessarily make him good, but it is pretty impressive.