He played a key role in the refinement and balance of the Basic Roleplaying system, which makes him one of the crucial designers of RuneQuest and Call of Cthulhu, as well as the other lesser lights driven by BRP. He also co-designed the Ghostbusters RPG, which is the second-best licensed RPG ever created, and incidentally provided the die pool architecture for Shadowrun and for the White Wolf Storyteller engine.
He also played a key role as shepherd and guardian of Call of Cthulhu for its first twenty years. My personal favorite edition of the game, the Fifth, is his final statement on what the game should contain, and contains probably more Lynn Willis text than it does Sandy Petersen text; Lynn's text amplifies and illuminates Sandy's brilliant core while leaving the central game design clear and explicit. Indeed, much of the moral power of Call of Cthulhu comes from Lynn's careful writing; much of its vast flexibility in play style and even adventure approach flows from the books he edited in the line thereafter. His last great Call of Cthulhu product as Line Developer was Chaosium's last great Call of Cthulhu product: Beyond the Mountains of Madness. Every Call of Cthulhu product you've ever loved from the first edition to 1999 exists because Lynn Willis made sure its text was coherent, made sure its gameplay was sound, and made sure it got to print. And there's a strong possibility that the thing you like best about it was added or correctly shaped by Lynn, not by the credited author.
An enthusiastic reader with a wide range of interests, he also helped inaugurate and shape Chaosium's fiction line, which by itself probably saved the company at least once.
He didn't do the convention circuit, so I only met him a couple of times, at the Chaosium offices. He very much liked a few ideas I pitched him, but sadly my pitches coincided with one of Chaosium's dry economic spells, so we never got to produce the massive Undying Mars campaign setting we both dreamed of. He was a gentleman, funny and interesting, and I regret the emails we never exchanged and the talks we never had more than I regret the game book we never produced.