He is most famous for developing the 2 into 1 exhaust system,
first using disk brakes on bikes,
first using automotive 'mag' and aluminum rims on bikes
and being the first guy to run a primary belt drive on a Harley V Twin made by using belts from the 6-71 dragster blowers on his own machined pulleys in 1963.
He actually came up with a lot more than these few items and invented countless 'small' innovations for bikes that have become almost commonplace today.
And he was probably the first to make an aftermarket, narrow springer, from 3" to 36" longer than stock. Just like Meyer's Manx dune buggies, there was far more demand, and several companies went into business making similar but not as high quality.
Dick experimented, improved and then built Choppers so that other people could ride and enjoy having a properly modified bike and not some monstrosity built by a celebrity. He cared not for fame or fortune and sought out neither during his lifetime. He didn't run a shop that sold glitz and bling for inflated prices. He didn't rip people off for 'services' and basically he lived by the 'Golden Rule' of Do unto others as you would have them do unto you'. Dick was into bikes for the pure joy and excitement of the 'Ride' and this is probably the real reason he became a legend. Somebody in the industry was once quoted as saying that Dick's 'Word' was better than any printed legal 'guarantee' offered by any manufacturer anywhere at anytime.
http://www.chopperhandbook.com/allen1.htm inspired by a story in an issue of Garage Magazine, issue 11, October 2006