Leach was transformative for the sport. 1990's college football was basically pound the rock, punt for field position, and hope you win 16-13. I would argue that Leach and Spurrier were the two guys responsible for the way college football is played today. Leach's coaching tree is ridiculous.
I'm not sure we can say that about anyone else in college football right now (or over the last 20 years). We can point to a guy like Saban and all the success he had - but he's not really reinventing the wheel - he's just doing everything at an elite level. Saban's records will stand, but what impact has he had on the sport?
On top of that, Leach was fun. I've tuned into games at TTU,WSU, and Miss State that i'd otherwise have no interest in specifically because of Leach. His offense was fun, his personality was fun - this is an entertainment product, and there wasn't a guy who gave you more than Leach.
I bounced around some message boards, and I can't recall so many positive feelings about any other coach from so many different fan bases all over the country.
The entire sport will miss him.