I'll admit, I'm not a huge John Madden fan. I'd prefer to have Chris Collinsworth or Ron Jaworski calling games. However, my dislike for him as an announcer is the exact reason that makes him the best football announcer ever, he made the very complex, very simple.
He coined phrases like "in the box", and the "red zone," which to casual fans, explains why teams may run or pass the ball, and showed the emphasis coaches place on key parts of the field.
Thanksgiving football games became traditions in many households because he talked about about football in a way that engaged non-fans of the sport.
He democratized the sport by creating heroes (Brett Favre), legends (Joe Montana), that people could relate to. He basically invented the telestrator, showing the casual fan what these heroes saw, and what made them so great.
In essence, he was the gateway drug for many people into the sport, and the NFL's rise in popularity can probably be attributed to him as much as any other one person.
I'm not going to miss Madden calling games, but I will miss the legions of fans that started watching the sport because of how accessible he made it.
The NFL is certainly in a worse place today because he retired.