Obviously the man was a titan in sports video games. Not only did he lend his name and commentary to one of the biggest franchises in the world (possibly the biggest game franchise named after a real person; unless you lump in all the Clancy games together) but he helped design the initial versions of the game and make sure it was realistic and true to football.
Like a lot of people I no longer play Madden, but I did put tens of hours in every year for like a decade and a half and I loved a lot of those games. I was a real football fan back then too, but Madden will always be associated with the video game that bore his name, and presumably will continue to do so now that he's gone.
I'm sad to hear of his passing. He was a unique and eccentric fellow, criss crossing the country on a bus because he was afraid to fly and introducing all of us to turducken and other culinary delights. Rest easy, big John. You earned it. You'll be telestrating plays in heaven from now on.