Rumsfeld was complicated, but there's no doubt he was very effective at leading the Department. I think most people fail to realize how sophisticated the Office of the Secretary of Defense is. Their resources reel the mind, most of all the human capital, many with PhDs, many very savvy political operators with stunning operational experiences. As a small example, as I recall, Google's hallowed SRE system was developed by an engineer who had come up through the ranks of Navy nuclear power. That's but one small component reporting into OSD.
Not a Rumsfeld apologist, by any means. Errol Morris did a good job showing the man for who he is, and it's not pretty (1). But reading HN comments opining about the leadership qualities of a Navy fighter pilot who was both the youngest and oldest SECDEF makes me realize how the Internet lets people indulge in a Dunning-Kruger situation the likes of which humanity has never seen.