Determination. Because of this determination, he was able to hone his skills and develop in the harsh environment of the middleweight division of 70's. Determination to withstand the assaults on the psyche that two straight losses against Philadelphia's best where others might falter and decline. Determination to isolate himself during training in his own self-made prison well after he had won the title, somehow maintaining the hunger after finally hitting the big time. This determination bleeds over into all the other aspects of what he did to make him the all-rounder he became.
Along with determination, I would say that him taking every fight seriously separates him from a lot of fighters (even some all time greats goofed around or didn't train hard for some fights). He was always in shape, yet he didn't overtrain and let his body break down. That's a wisdom for life in general, not just fighting.
Professionalism, always in top shape, even between fights, dedication to a Spartan lifestyle, willing to go to opponents home town , possibly, probably the greatest switch hitter in the history of the sport. In his prime he could go from orthodox, to south paw, to orthodox so smoothly it was hard too see he had switched, even on tv. Imagine what his opponents felt like.
Everything. There are few, who develop to a level of acquiring Hagler's all-round game, which was forged in the heat of his experiences, using his innate talent, athleticism; a killer instinct and a will of steel.
At his peak, Hagler's great strength was that he didn't do anything badly - in either stance. Whatever an oponent did well, Marvin could neutralise and counter. Have a look at the Tony Sibson fight. In the 3rd round it looks like Tony is getting on top, Drawing Marvin in and banging in hooks. In the 4th, having had a good look at his challenger, Marvin switches stance and, as he put it, " Started dipping into my toolbox ". There are very few Middleweights that could have set insurmountable problems for Hagler at his best.