There was a great deal of innovation in Dungeons & Dragons when it finally hit the market in 1974; D&D essentially created the idea of tabletop roleplaying games that we know today. However, as with a lot of ideas it went through several stages of evolution, evolving from tabletop miniatures wargaming to something similar to squad level skirmish wargaming to individual players controlling individual characters. As Gary Gygax explained it in an interview:

This goes all the way back to Gen Con 1 with a fellow by the name of Jerry White from Portland, Oregon, as I recall. He brought a bunch of 40 millimeter Elastolin figurines done by Hauser. I started playing and got hooked. Jeff Perren happened to collect quite a few of those same scale miniatures and he had gone and set up rules for them. Henry Bodenstadt ran a gaming shop business out in New York, he had got his "Siege of Bodenstadt" game into Strategy & Tactics magazine at that time published by Chris Wagner. This was what really hyped the 40 mm figurines.

Playing games with those miniatures and a castle, hooked me completely on such gaming, so Jeff Perren came to Lake Geneva and I had a sand table in my basement. He brought in his 40 millimeter Elastolins and his set of rules where one figure equaled 20 men, and we both had a heck of a time, but he lost interest later. And he left the rules with me and I said, "I will expand these, they need to cover more historical settings." I turned two pages of rules into about four. And then I figured it would be a lot more fun to play man-to-man and make those weapons count. So I wrote up a set of man-to-man rules, then rules for jousting, and finally a fantasy supplement with dragons, heroes, magic swords and spells; and eventually those were published as Chainmail. [...]

Well when you were one-for-one in most areas, you had an Elastolin figure; so the player would control any figure on the table that was his. And each one would take two hits to kill or whatever. Dave Arneson up in the Twin Cities started a Chainmail campaign where all the players each had one figure and he brought that style of game down to Lake Geneva to show me. I said, "Wow, that's really cool". So out of his inspiration, I created the D&D rules.

That's last part's a bit of a not-telling-the-whole truth on Gygax's part, since Arneson came up with a lot more concepts than just one player, one character, but Gygax ended up writing the rules.

While there are some things we can say now were a precursor to roleplaying, none of them were widespread or commercial in the sense that D&D was. Little Wars for example, was a miniatures wargame that was created in 1913 by H. G. Wells and used toy soldiers, but was much more whimsical and less fantastic than Chainmail. In an interview Fritz Leiber talks about how he and his friend Harry Otto Fischer created a kind of miniatures board game based on their Lankhmar setting in the 60s, but it was never publshed until much later (and that, through the office of Gary Gygax); Leiber of course was the author of the Fafhrd & Gray Mouser stories, which were a substantial influence on Dungeons & Dragons.

So in a very real way, I would say Gygax and Arneson were innovators. They weren't creating things out of whole cloth - the system evolved over time out of miniatures wargaming rules, and the setting materials borrow liberally from dozens of pulp, fantasy, weird, and science fiction writers, from Tolkien, Michael Moorcock, and A. Merritt to Jack Vance, Robert E. Howard, and H. P. Lovecraft - but what they finally put together was unique.