Arneson was a driving force behind D&D's creation and his contribution to the world of adventure gaming should not be underestimated. It was Arneson's spark that transformed Gygax's game Chainmail into the first edition of D&D, and begat everything that followed.
Arneson had to fight to get credit for his contributions, filing multiple lawsuits (later resolved out-of-court) against Gygax over crediting and royalties. He nonetheless did return to TSR in the mid-'80s to work with Gygax again. Following that, he began a second career as an educator, working in several schools with a particular focus on how to use gaming as an instructional tool.
You will be missed, Dave. *pouring out a little D20* Save a dragon for me.
Thanks to JMR, who once rolled snake-eyes on two D20's like five times in a row.