It's amazing that someone who hit as hard as Ray Nitschke managed to have a 15 year NFL career. When I read about him, he's usually described as "tough" or "ferocious" and a great hitter and tackler. Nitschke said he loved contact. I've also read that he was wonderful and caring person off the football field. His on field transformation into one of the toughest guys in the NFL was what Bart Starr called "a classic example of Dr. Jeckle and Mr. Hyde."
While he was only named to one Pro Bowl, and only twice named first-team All-Pro, he was also named as the top middle LB on the NFL's 50th anniversary team in 1970, and the 75th anniversary team in 1994. He was inducted into the Pro Football Hall of Fame in 1978, as soon as he was eligible after retiring in 1972.
NFL.com has a 5 minute video remembering him.