You guys keep reducing Arneson's contributions to just an idea. He ran the games, he gave his notes to Gary, he came up with so much of it. He wasn't just "the idea guy."
I think this persists because Arneson's contributions could have been so much more. Charged with design at TSR, Arneson never produced any large coherent documents. His Blackmoor adventures were lightly regarded and made for strange reading (and co-authored to boot), and at least among the circles I was in were considered far less interesting that Greyhawk and far less playable than something like 'Keep on the Borderlands' or 'Tomb of Horrors'. We wanted more of what Gygax was writing, and Gygax was writing.
It's hard to know what actually happened, but all the sources seem to agree that Arneson was just not producing content at TSR. His notes remained cryptic even to his peers. Few pages of copy were coming from his desk. No revolutionary designs were forth coming. For all the fact that Arneson was the first Game Master in history, and got there a year before Gygax, it's hard to point to what he did in the industry other than create the idea for it. I think if he wrote one module that everyone wanted to play and everyone still talked about, things might be entirely different. For whatever reason, stuff with just his name on it wasn't coming out.
And, this is a bit of speculation on my part, but I think Arneson never really wanted to write or play RPGs of the sort that he created and which were grabbing everyone's imagination. A staunch wargamer who was deeply involved in the Strategos N scene, I very much get the impression that he was deeply frustrated by the fact that his players abandoned the game he wanted to play, which was a lavish domain management game that would generate wargame scenarios, and were instead leaving all to participate in what he considered a mere minigame of exploring the dungeons of Blackmoor. His campaign as he envisioned it of players rallying together to fight off invading armies, were falling apart because the players would rather kick the doors down, slay trolls one on one, and take their stuff. I always envision in my head something of the dynamics of The Knights of the Dinner Table, as the artistic D.A. is continually frustrated in his grand design by the fact that his players are hack and slashers. But unfortunately, Arneson never produced that visionary dynastic domain management mass combat supplement for what would now be called paragon tier play, and so what we remember from the early 80's is kicking down the doors, killing things, and taking their stuff. By the time we were ready for something like BattleSystem, Doug Niles was doing the writing.
The truth is, his job at TSR was to be 'the idea guy', but if you never really do anything but run games, give your colleagues a few cryptic badly organized notes, and never actually produce a lot of concrete writing, you are barely even the idea guy. In the last 4 weeks, I've written more pages than Arneson produced in his entire time at TSR - and have coded one set of rules into application to automate it. Now granted, I have the benefit of a modern word processor and 50 years of rules innovation to draw from, but if I were in a small business in the 1970's, and I'd bet my livelihood on the businesses success, and my partner was Arneson, I think I would have been frustrated as well.
Again, this is not to say Arneson doesn't deserve full credit for the invention of RPGs, but well, I think he was mostly an idea guy.